Clutch needs bleeding, master or slave is faulty, clutch fork is bent/damaged, clutch plate is bent, pressure plate is faulty/damaged, release bearing is too short. One of those lol. Shorten the list by answering this: Why did you change the box? Did you change the clutch/pressure plate? Did you use the release bearing from the old box? Did you disconnect the fluid line from the slave cylinder when you removed the box? Is the slave cylinder leaking at all? (Pull the boot off to have a closer look, should be dry)