when tranny hot wont move when cold will move

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when tranny hot wont move when cold will move, just bought it. the guy said was slipping, they changed transmission filter and serviced and it worked fine. I bought it yesterday drove it home, parked got up this morning warmed it up and it wouldnt go in any gear. turned it off came back in a couple hrs started it and put it into gear(cold) and it worked. what could be the problem?