Can't add markers to SOME subclips but can add them to others

Question

Hi. First time poster, I hope I'm not accidentally posting this in the wrong section. I have scoured the discussions for this problem or I wouldn't dry your eyeballs with this.
Suddenly, right in the middle of editing, I can no longer ADD markers to some specific clips in my bin. It is the oddest thing. In anticipation of questions, I'll give every detail I can conjure, I hope I'm not being too verbose but it's a very puzzling situation.
- The "problem" subclips, to which I can suddenly no longer add markers, at one point had completely normal marker functionality. There's pre-existing markers on these clips that are now no longer editable (with the M or ` key), and every marker menu item under mark -> markers is greyed out, EXCEPT for "add" and "delete all..." neither of which work on the problem clips anyway. These are all functions that work as usual on the non-problem clips.
- Within the problem subclips, I can still navigate to the next and previous markers, with shortcut keys and menu commands. But when I hit M on a marker in a problem clip, nothing happens. And when I hit M in a "blank" space in a problem clip, again, nothing happens, i.e., no marker is added. Nor do the menu commands pertaining to markers function.
- the clips in question are subclips, but so are the clips that have absolutely no problem with marker functionality.
- the clips not having this problem, which outnumber those that do, are not only in the same format, same media, same compression, from the same tape, captured in the same session and on the same drive but are even from the same "master clip" as their problematic siblings.
- the master clip in question has normal marker functionality.
- brand new subclips from that master clip have normal marker functionality, even if their in and out points are identical to the problem subclips...but then again, that's not surprising, since the old ones had "normal births," too.
- if I copy and paste the problem subclips in the bin, the copies have the same problem.
- if I "duplicate as new master clip" the problem subclips in the bin, the duplicates have the same problem.
- I can still edit the markers on the problem clips by right-clicking the drop-down list of markers IN THE BIN and selecting "edit." I can also delete them there by selecting them in the bin and hitting the delete key.
- doing so does not enable me to add more markers. Which reminds me:
- I am not doing something weird with markers - even if there was a quota on markers, I imagine I'm nowhere near it. it just seems like I'm the "crazy marker guy" because you're reading six pages of me talking about markers. I'm not the crazy marker guy. I'm an ordinary man with a crazy marker problem. I use markers the way you do. I mark the beginnings of takes and noteworthy reactions, etc....there's maybe an average of 15 note markers in each subclip, but plenty of "functioning" subclips from the same master clip have more markers than the problem ones.
- yeah, I quit and restarted FCP. And my computer. Yeah, my "M" key works (look how many times I've used it), My mouse is plugged in, I combed my hair, my software's not stolen, I purged my render files and, believe it or not, I only had one coffee this morning.
- I haven't been working with FCP so long that I can say I have or haven't had this problem with non subclips. I've only done a couple projects with FCP so far...this one is not unusual in comparison, but this is definitely the first problem I've ever had with FCP.
- If you have read this far, you are hero not just to me, but to all of humanity.
- if you have the solution, you are a God or I am an idiot, I am praying for either to be the case and I'm not picky. Seriously, thanks in advance to anyone that wastes their brain on this.

Answer 1

Figure I may as well update this thread with the different things I try. I know this makes for compelling reading and I don't want to deny you the story.
- I deleted the Final Cut plist file from the preferences folder, which not only had no impact on the problem, but no apparent impact on Final Cut, which is weird, but, one riddle at a time.
- I went through my autosave vault on the scratch disk, which deepened the confusion for me: each of the three problem-clips had a specific time before and after they became frozen to marker activity. I loaded the autosaved project from 11 am, for instance, and apparently, at that time, the most recent clip to become marker-locked was still working fine, but the other two weren't. A few hours earlier than that, only one of them had the problem, and at about 6:30 this morning, the problem didn't exist.
An important thing to note is that the order in which these clips became marker-locked follows the order in which I went through them, marked their takes and moved on.
I didn't notice the problem with any of them until much later - when a creative shift prompted me to add some new marks, at which point I realized that for three of my clips, I couldn't.
So, if this problem is being triggered by me, it's happening not during actual marking (I would have noticed), but during the things I do with marked clips, namely shuttling through them with the JKL keys, jumping to each of the markers, setting in and out points and inserting them into the timeline.
Is there some key it's possible for a less than attentive, sleep deprived, FCP newbie to hit, during that activity, that "locks" the current clip to the addition and adjustment of markers? As sensitive as my ego is, I really want this to be my fault so I can learn the problem and avoid it forever. I don't want to think there's some supernatural time bomb glitch in the one copy of FCP I bought that causes random clips to slowly dry like cement after which they can never be marked again.