The first thing I'd do is thoroughly clean the tape path and try a different tape (particularly if the other tape was Ampex 456).
Tapes with Sticky Shed can slow or even stop a machine dead in it's tracks.
While you're cleaning the tape path, make sure that all the rolling guides are turning freely.
Next, check to be sure that one of the brakes isn't dragging. If your machine has an Edit function, it should disengage the brakes (and pinch roller). With the brakes disengaged, try turning the reels (you can do this without any tape threaded). If one of the brakes is dragging, you'll feel resistance as you turn the reel or reel turntable.
If that's the problem, examine the brakes. They may only need to be adjusted.