Can't shift into gear when car is running only when car is off.

I replaced my transmission with a used one and I also
installed a new clutch kits which included a clutch disc
pressure plate slave cylinder and master cylinder. This isn't
my first time putting in a clutch kit in this vehicle so I Kno
I'm doing it right. I did however replace my oil pan gasket
for the first time during all of this and had to lift my engine
to get pan off. when putting screws back in the motor
mounts after finished they were very hard to turn to get
back in. Now that everything's installed when I start the car
I cannot get it to go in gear but when the car is off slides in
the gear easily. I had a buddy set in the cab push on the
clutch while I crawled up under the car and looked in the
transmission. When pushed the slave cylinder is extending
and pushing in the clutch forks but pressure plate isn't
releasing the flywheel. Not sure if slave cylinder isn't
extending enough or if clutch just isn't working properly.
And clutch disk is not backwards either. Plz help!