After vehicle has warmed up transmission is hard to shift and grinds gears

Hello, I'm Danny.

You didn't really specify how many miles are on the transmission but I would suspect the synchro's are wearing or worn out. Each gear in the transmission has a synchro along with it to act as a buffer when shifting between gears. Also, you might notice it more once the car is warm due to the transmission fluid getting warmer with temperature and then the oil is more thin creating less of a buffer. Hope this helps and thanks for using 2CarPros.