1994 Nissan Truck Clutch doesn't engage when running

I just replaced the entire clutch system: clutch disc, pressure plate, release bearing, master cylinder, slave cylinder. I bled the system, and when the car is off, the clutch works perfectly. Once the car is started, the clutch engages at first, then gets progressively worse until it doesn't seem to engage at all. The only part of the hydraulic system I didn't replace was the damper. Would that cause this problem? I've bled it three times, at both the damper and the slave cylinder. Any ideas?