Which resolution are you trying to lock - CGA, EGA, or VGA? The problem with the D9200's is that it often gets memory corruption, and stops recognizing the resolution being input, and gets stuck in one resolution mode.
Now, this is easy to fix, but only by accessing the on-screen menu. If you can't get the menu up because the picture isn't stable, then you may have to connect it to a video signal of a different type first. Once you can get the menu up, exit back out, then hold the DOWN and SEL buttons down at the same time. After a few seconds, another menu will pop up that looks like the other. This is the Factory Menu. Do a Memory Recall. Now exit out of the menus and try your current setup. It should work now.
On another note - if you are using this as CGA or EGA, and have the sync connected to both the horizontal and vertical negative sync, you should disconnect the vertical and try again...