My '63 SIIA has developed and interesting issue. After driving for a while I cannot select 1st and 2nd gear while the car is stopped (engine running). No grinding, just the gear lever refusing to get into these two gears. I can select 3rd, release the clutch momentarily and then shift to 1st and that works ok. Interestingly enough, I have no problem shifting into 1st or 2nd for the first few minutes I drive the truck so something happens when everything warms up (which is somewhat counterintuitive).

Any thoughts as to what may be going on?

Thank you
Vangelis