My car wont go in first gear when it warms up

I start my mav and it goes in all the gears and drives fine but once it warms up I try to put it in first and it wont go so I have to start in second.It also grinds if I put it in reverse when its warmed up.I was told it was the clutch not the tran is this true?what would cause this? its a manual column shifter in a 72 302 mav thanks.

 

have you checked the gear oil an or changed it?

 

I have yes that was my first thought too.