Thanks so much, Andres-
Late last night I managed to get it to work by primarily by increasing the magnitude of the stars used from 12 to 14. From your points below that would make sense since it is a fairly deep, stretched image that you can see at (double click on it to get downloadable one):http://www.astrophotogallery.org/member-galleries/p9493-m51-lrgb.html
Although I haven't yet gone back to try it on the previous ones which were subs for this image, I presume they would require the same modification, although they were linear, non-stretched.
Also another key thing (probably obvious) was that ImageSolver did not work on this image directly (the posted JPG, or 16 bit TIF from Photoshop that I use for printing at home and posting jpg's). In fact, it wouldn't work when I just saved a new FIT file from the TIF I got back into PI from Photoshop. To get it to work, I had to go back to the original FIT file from PI 1.7 that was used to generate the posted image made in Photoshop. The images are nearly identical (but full-frame with less cropping) but presumably even the remade "FIT from TIF" doesn't support or contain the WCS, or other FIT headers needed by ImageSolver and the annotation script.
I believe that I had the coordinates (for m51) correct and image resolution/scale correct from the previous successful solutions, and I think with this image of almost 8 hours of integration time that it shouldn't have been noise that created problems, so I am puzzled why the stars display seemed to show so many +'s that didn't appear to be connected to stars in the image. Nonetheless, when it finally worked using an ORIGINAL FIT and using mag 14 stars, the result gave identification (with all the databases turned on) to almost all the visible galaxies and other objects (except for the 2 little edge-on's near m51B that must not be in any database). In ALL cases the markings were right on top of the features visible in my (full frame) image so it looks there was no significant distortion that might have been causing troubles.