clutch help please

[chopsticks]

Alright so I put in a new motor last week in my car and it runs good. but im having a problem.

When the car is off i can push in the clutch and go through the gears with no problem. But when i start the car and try i cant get it to go in any of the gears.

iv tried bleeding the clutch, checking the master and slave both for any leaks . the fork is in the right place also, And Im still having this problem

I don't really know what eles to do. the clutch was replaced maybe 300 or so miles before the motor was replaced so its still in good shape.
Has anyone had this problem or know what i could check for or do to fix it?
any help would be appreciated!!!

thanks michael