Are you saying that it appears to have a limit, on the number of pictures you can have on the phone? Do you have at least several GB of memory available to add pictures or apps to the phone?
If, for example, your pictures are in folders under "My Pictures" and it is labeled "Cars", you can't add another folder under "Cars", they won't transfer to the iDevice. Under My Pictures folder you could have "Cars Chevy" and "Cars Ford" folders for intense, but you can't have folders within folders under "My Pictures".
Have you tried a reset. Close all apps then - Hold down the HOME, bottom button, and the SLEEP, top button, for about 10 seconds until the Apple icon appears, then release buttons. Allow it to reboot, it may take a few minutes, don't panic like I did, wait for it to reboot. Cures many weird problems on iOS devices. NO this won't lose any apps data etc.