97ramairtransam

the other day i was goin down the road and i tryed to put it into 2nd and it wouldnt go it acted like it was slipping.my car has a 2800 slp stall and a trans go sift kit and if i take off in over drive it shifts great and hard in all gears but like i said it wouldnt do it manualy..ps could it be that the car has been sitting for a year without being driven?