First thing that comes to mind is does it say "SPL-0.57.CMONEX" on the boot loader screen? Dont take that as an insult but it will say Security Unlocked even when the phone is locked.
Second guess would be that (I know you have CDMA since you said sprint) you are trying to install a GSM rom on a CDMA phone.
Third guess is that you are trying to install a rom with unsigned spl in which case you have to use the
hard spl developers edition
Anything other than that and I would need more information about what it is your doing. Hope this helps ya.