If you have the 4G, DO NOT put on the non4g rom. You'll brick your phone.
Lets try and figure out what's going on. Now, I'll probably ask you some really basic questions, but often I find it is the simplest things that are the problem, because they're so easy to overlook.
Do you currently have the phone unlocked?
What recovery do you have installed on the phone?
What methods are you trying to flash the newer rom on?
I've seen a bunch of people with issues while trying to use a ROM Manager, and had issues. On other phones (I'm new to the Crespo4g), after a failed flash, people would often revert to their stock ROM, which would relock things. Also, if you're using stock recovery, that may give you problems (try Clockworkmod).
I just got my phone this week, and when I got things setup (Clockwork Mod with CM9 nightlies), it was really painless. I followed the instructions on the Cyanogenmod wiki (just using the nightly rom and ICS Gapps inplace of the CM stable rom the wiki recommended). I'd recommend trying that step by step, and if there is anything I can do to help, let me know.
