Sounds like my OD. Does it work when its cold? Does it work at higher speeds? It could be a clogged filter or bum solenoid, but most likely the seals are gone inside the unit. Then the unit can't make enough pressure to shift. If this is your problem it tends to work when its cold I asssume becuase the oil is more viscous, and it works when going faster because the pump is pumping faster.
I've already ruled out electrical problems on mine and I'm going to have a go at putting new seals in it. I have seen one apart at the junkyard and it does not look too bad.