Mancini transmission

(Pan on)
Fluid level
Linkage chk
Mainline pressure test.

(Pan off)
Correct filter
rear band adjustment and servo chk
Valve body torqued

It's possible that the seals sat unused for so long that they hardened up, but it's a little odd that it engaged before and now wont.
That pump pressure test would be interesting.

One other note,
There is a big difference in not engaging at all and not shifting.
The valve body will not cause a non engagement issue,(except for lingage ) but could cause a no shift problem.