In most software, Find Next and Find Previous operate from the current position of the cursor. So in MATLAB, you can Find Next a few times down the file, then click back at the beginning, and it will start searching from the cursor position.
GMAT doesn't do that. If you Find Next down to the end of the file, then click back at the beginning, it stops finding any matches entirely. You need to Find Previous all the way back to the beginning first, or close the dialog and try again.