t5 trouble (whats new)

i just bought another stang today and i have an issue with getting it to stay in first gear sometimes u can get it to go right in and stay in other times you play hell i dont know a lot about trannys but from what ive seen it almost seems like a tweaked shift fork. Any input will be helpful thanks

i found out if you give it a good smack into first it will stay in if not as soon as you let the clutch out grinds and pops out f that helps