When the car is cold, the transmission seems to go into all of the gears. As it warms up though, it gets harder and harder to get into some of the gears. Second gear always seems to work but the other gears are hit and miss. What is your guess before I take the car in? Thanks.

When was the oil last changed and what weight/type was used ?
With the correct oil the box should shift less easily until the oil has warmed up.

The transmission fluid was just changed a few months back

I'd check clutch adjustment. Make sure its completely disengaging and not dragging.

Good thought. Hopefully it's something simple like that or the linkage needing adjusting.

Please check my sig below for my 31pages of notes. There is info there about such things.

Transmission fluid goes into automatics, only use GL5 80w90 gear oil in your 901 transmission. Change every year, or sooner if hard driving/high miles.

It is not uncommon for things to get less precise when heated up as tolerances change. Oil was a good first recommendation as it changes as it is heated. Clutch cable is another culprit to verify. A quick test to know if your cable is too lose/not fully pulling/clutch not releasing, is if you grind AT ALL going into reverse. There are no synchronization parts on reverse and it is straight cut so it should slide right in with no issue unless it is still spinning due to being under load because the clutch is not releasing for one of the aforementioned reasons.

Of course, check mounts and bushings, always.

Miles on transmission? Any work done? Type of oil used?