this problem only occurs when car is cold. when it is warmed up it works fine.
why doesn't my car shift into overdrive when cold? on 1994 Toyota Corolla
It's not supposed to go into overdrive until the engine reaches a certain temp. to improve emissions controls. Is it taking longer for the engine to warm up? Maybe thermostat is opening to soon or is stuck partially open. If it concerns you , get it checked and verified before randomly spending money on parts.