very hard to get in gear when the car is on

[Pozobyt]

I just did a auto/manual conversion, and the car ran flawlessly the first few days, now all of a sudden, I can't get the car into gear with the car running. when the car is off, it goes into all gears very easily. the clutch feels fine, and I can watch the slave cylinder and it has the same ammount of movement as before when the clutch is engaged.(if that makes sense) I used a brand new oem clutch and throw out bearing. sometimes if I try to get it into first, it grinds slightly, even with the clutch pedal pressed to the floor.

I can start the car in first with the clutch engaged, and it's will run in first just fine, but still wont shift to second.