Clutch is definitely holding on. Don't mess with the gbox, it's nothing to do with the issue. As johnboy said above, could be the pressure plate on the clutch, or it could be a hydraulic issue.
If you pump the clutch quickly 5-6 times then hold it down does it go into gear easier? If so then your looking at something on the hydraulic side, if no different then chances are it's a mechanical issue with the clutch