1991 Other Isuzu Models Clutch gears will not engage

Transmission problem
1991 Other Isuzu Models 4 cyl Four Wheel Drive Manual

I have a 1991 Isuzu Pickup 4 cyl 2.6L 4x4. When the truck is not running you can easily engage the clutch gears. However; when you turn the truck on: the clutch will not allow you to engage the gears.

I have replaced the clutch plate, pressure plate, throw-out bearing, as well as the master clutch cylinder and slave clutch cylinder. Everything has been properly reinstalled and the clutch line has been bled.

So once again; when the engine is off you can engage the gears on the clutch; however, we turned the engine on and the exact same thing happened as before: you cannot engage the gears even with the clutch pedal fully pressed to the floor.

Please help!