VirtualDub output file doesn't play (it's nnnn frames of one image)

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I'm a newbie, and was "just" hoping to do some quick carving up of some avi files I captured with a new dashcam toy. Sure, maybe some day I'll take it to the next step and 'Cecil B. DeMille' it. But for now...

I downloaded VirtualDub 1.10.4 from Sourceforge, and after scouring the Interwebnets, help files, and this forum, managed to get it to actually open my avi files (they use H.264 encoding). I then read more help files, and a great newbie tutorial on basic functionality here so I knew how to operate the buttons. I then adding Mark In and Mark Out consisting of about 750 frames on my raw avi, and proceeded to "Save As" to a new avi filename. When I play the new avi file, it's just a single (first) IMAGE repeated for each of the 750 frames. In other words... it's a "video" of only the first frame. I'm just playing it with Windows Media Player, and the player appears to be working correctly (progress bar moves, can be paused, backed up, etc.). But that's it. WMP plays other avi files (like the raw one I'm trying to edit) just fine, so I don't think there's a player problem. Also (and I'm just guessing since I don't know how VirtualDub is supposed to behave), I don't get any "play" functionality within the program. I can manually advance frames (ie, the Forward and Backward buttons to see the "video" progression in VirtualDub... but only when I freshly open the raw avi. Once I add the Mark In and Mark Out, I no longer can even manually change the image (frame) displayed in either the Input or Output windows.

edit: When I run Tools | Test video in VD, it plays as you would expect. And I can create Mark In and Mark out on it, and play back the new section fine. There must be something wrong with the codec or settings or ???? for the raw video I'm trying to edit. But I don't have a clue what to look for.

edit 2: I noticed that, while I can manually slide the bar and watch frame by frame on my raw avi, I CANNOT get any frame movement to happen in VD AFTER I make any kind of edit whatsoever (ie, if I delete any frames, the program will no longer show any frame changes when I move the slider bar... it just stays on the one frame that was in the picture. I suspect this is the "source" of why my saved avi files don't have any frame movement in them... the question is WHY is this happening in the first place???


My efforts to find an Idiot's Guide for this application have failed me. So, I call on you... help the idiot, please