Why isn't my high definition video converting to standard definition?

Question

What I'm describing here, I've successfully accomplished many, many times using Premiere Pro CS5 and now Premiere Pro CS6. Now, for the first time ever, it's not working and I'm completely baffled. (This is a high-end Windows 7 64-Bit machine with 16GB RAM running Premiere Pro CS6.)
Here's a basic summary of my workflow. Nothing out of the ordinary here.
1. I shoot my video using a high definition camcorder (the same one I've been working with for the past year or two.
2. I import my high definition video into a 1920x1080 sequence in Premiere Pro CS6.
3. I edit my sequence until I'm happy.
4. I enter and name my Encore chapter markers in the sequence.
5. I create my Encore project and import the sequence as a timeline and set up my menus and such.
6. I build my Encore project, first into a standard definition DVD image file (so I can later burn SD-DVDs) and then build it again into a high definition Blu-ray image file (so I can later burn Blu-rays).
The Blu-ray burn is turning out just fine.
But the video in the DVD burn is all black. I can jump around from chapter to chapter and I can hear the sound, but the video doesn't show up!
I tried this a couple times thinking that maybe Encore just messed up the SD-DVD creation process but I got the same result each time. When I preview the project in Encore, it all looks just right.
So I figured, okay, something's not working when I import the sequence.
I went back to Premiere Pro thinking I'd just export my 1920x1080 sequence into a widescreen standard definition AVI and create a new sequence just for the standard definition DVD.
The exported AVI is black as well!!
I don't get it. Why on earth won't my high definition video convert properly to standard definition?
The high definition sequence looks completely normal in Premiere Pro. What could I possibly be overlooking??
One final note. Just as another test, I copied and pasted the sequence in Premier Pro, truncated it to a couple minutes (the full sequence is 90 minutes long), and exported to a widescreen AVI again.
It just finished, I tried playing it back in Windows, and it, too, is all black, sound but no video.
Like I said, this is nothing new for me. I've done it countless times with other projects. I could really use some assistance with this one.

Answer 1

So I exported my completed high definition sequence to an MP4 and re-imported back into Premiere Pro. Right-clicked it and created a sequence from the clip. Then I imported my chapters from the old sequence into the new one.
I returned to Encore, modified my project to work off the newly created sequence (what a pain having to recreate all the scene links) and voila, it had no trouble making a DVD this time.
So I guess I'll get out of the bad habit of my old routine and create a sequence the "right" way from now on. Funny how we get used to doing something one way and stop thinking outside the box after a while.
Jim, I don't see how that's necessary. As long as I create an Encore project based on a good high definition sequence, I have never had to do anything other than have Encore build a standard definition DVD based on the high definition sequences. It has always done the conversion for me and produced really good DVDs. Once I create a DVD image that I'm satisfied with, I simply have Encore build a Blu-ray image using the exact same project, and that works perfectly as well. I suppose I may be missing something here but it all works and works well, so I don't see a reason to create two projects when one works well.
On a separate note, is it just me or doesn't everybody think it's high time that Adobe offer a feature to import markers from one sequence into another. I can do it automatically but I have to use a macro that I created separate and apart from Premiere Pro. For that matter, I think Encore should also offer a feature to remap all scene links to a new sequence instead of forcing me to fix them one by one myself.
Anyway, thanks to all of you for pointing me in the right direction!