Grinds and wont go into 2nd gear

(96 mustang gt) when i try to shift into 2nd
gear clutch is engaged, its in the right
position it seems and feels like its there. I
start to let off the clutch it grinds
immediately. Previous owner could get it to
shift by pressing the clutch again after it
was in position and i guess hitting the right
rpm but would still grind, he tried teaching
me the trick but havent been able to figure it
out i cant seem to find the right rpm though.
Transmission was recently rebuilt when he
had it. New clutch new master cylinder new
shifter. Im stumped on this problem
synchros maybe???