I would start with the easy & work up
1. Check pedal travel, does it return fully hitting the stop.
2. Any creaking noise from pedal when depressed could be worn pedal bushes or broken pedal box?
3. Get someone to operate the clutch while you look to see if your slave cylinder is operating fully.
4. Bleed, careful if you have the damper pipe connected? Bleed nipple at rear tends to seize & you can fracture the pipe.
5 could be your gear box oil as suggested,
6. Change master & slave cylinders.
I done all the above, then tweaking the adjustment over a few days until I was happy.
Sent from my GT-N7100 using Tapatalk