[olddog]

The clutch is not disengaging or the pilot bearing is dragging. The synchronizer is unable to stop the input shaft from turning, therefore the clutch or pilot drag is stronger than the synchronizer.

As stated, low fluid level, boiling fluid, bad master cylinder, bad slave cylinder, air in the lines, are all possible. If you do a search, you should find approaching a million threads on this subject. Exaggerated? Yes! There are tons of information already written on this.