[dap930]

If there has been no clutch replacement recently, then the clutch cable probably needs adjustment under the transmission. If the clutch does not release fully, the disk can drag enough to keep the transmission gears turning, particularly when the transmission oil is hot. If you can move the shifter towards first or second to engage the syncros then quickly go for reverse, sometimes you can get it to go in.
If the clutch has been recently replaced it may need more time to seat in to wear the high spots off the disk, but it may still need a cable adjustment to get more release travel.