I couldn't find this mentioned in the archives. After returning to the car from a shopping stop, the transmission would not shift up from 1st gear. When accelerating up, then releasing accelerator, car coasts - cannot detect any engine drag. Pushing back on accelerator while coasting, engine revs up until the transmission engages. Thought fluid might be low - didn't look that way, but added some. Definitely overfull now, still stuck in 1st gear.

What is the likely diagnosis?

Thanks. Steve.