'Group in stack' command fails with selected photos in Lightroom 1.2

Question

I am a Mac user (new Intel iMac running OS 10.4.11), and have successfully been able to stack photos up until this point. Now suddenly, they will not stack. I've color labelled photos in a shoot by theme, and have been stacking them. I can't see anything different in this situation.
I tried closing out LR, then restarting. Stacking still doesn't work. (Options in the Photo > Stacking menu are greyed out.)
Any suggestions?

Answer 1

I am not in a collection, but your question prompted me to notice that while all of the photos I am working with at the moment are from a single shoot, and therefore should be in one folder, for some reason, not all of them were showing up in the folder. I spent considerable time yesterday trying to figure out a way to locate them, and I am suspecting that I inadvertently made a mistake somewhere with keywords. I do recall seeing an unusual icon in the upper right hand corner of one of the images. When I held the cursor over it, it opened a window that told me that the metadata had been changed outside of the program (!?) and how did I want LR to handle this. I had no idea what this was referring to, how it happened, or what the implications were -- but I had to make a choice, so I chose 'overwrite'. It may be coincidental that the stacking problem started about the same time. In any case, I finally got them all back in view, and systematically went back through them making sure that each batch had the desired keywords. In the mean time -- for whatever reason -- I am once again able to operate stacking & its functions.
As you can see, I am still very much a beginner with LR, but as I go, I am loving its power and generally more intuitive feel. I am using far more than PS at this point, although I know that there will be some things I can only do in PS.
Thank you for your support!