Yeah, I had some big time prostate issues for about a month after my vas. It was really hard to piss and really hard to keep it going. That symptom did eventually subside, but my pain did not unfortunately.
IIRC the prostate and pelvic floor have to relax to allow urine to flow out of the bladder through the urethra.
If the prostate is inflammed or your pelvic floor is in spasm due to nerve damage, pain, whatever, it's gonna be hard to start the flow.