95 Altima manual transmission

My 95 altima will not go into gear when engine is running, but will go into any gear when off. Had a new clutch installed approx. 6 months ago, and has been running fine. Help!!!

not really sure man but i have had four transmissions put in my 95, syncronizer will go bad on them and sometimes they get stuck in gear it sucks when that happens could be a hydralic problem check the fluid or ask cottman transmissions n e ways later, oh yeah try to keep down on tha racin aspect or u might break a ring gear like me lol