are you saying that when you put on a condom your penis becomes flaccid? or did you start having troubles after you put on a particular condom? the lack of a large vein you are used to feeling could be due to a decrease in blood flow - or erectile dysfunction - or anything that might prevent your erection from hardening fully.
i try to put on a condom and then penis when flaccid. i never had sex before. i'm 29. i used to feel a larger vein that i don't feel anymore.