The 1st and Reverse do not have a synchro on them, so my guess is the teeth are possibly damaged on your Reverse, the reverse slides on the 1st gear as I recall.
Other than making sure your clutch is in decent shape, you might need to check that Reverse gear or the gear on the counter shaft. Often when a synchro sticks it can be the shift fork, and can hard to get out of gear. Since there is no shift fork for non-synchro gears, you don't have too much else aside from the gear.
