...when the engine is cool, the clutch engaged and changed smoothly, but after driving a while it become hard to shift gear..like the clutch is gone...any advice?
May I suggest some possibilities based on my own and my relative's experience:
1. Perhaps the clutch master/slave cylinder seals are on their last leg. When hot, it
won't hold pressure.
2. Dirty clutch(brake) fluid which has absorbed a lot of moisture.
3. Gear oil issues. I've seen many using GL5 oil instead of the correct GL4 fluid for gearbox. GL5 is for rear axle only. There are many articles online on this issue.