Trans problem

[C8 route66]

I have an '63 with a power glide that has issues with shifting. It upshifts at around 20mph (~30 kph) regardless of how much throttle. It does not downshift, passing gear, unless it is shifted by the gear lever. Any ideas as to what the problem is? Ia there anyone that still works on power glides north west of London, ON.?

Thanks, John