TF 727: no movement when select manual Low (L or 1)

Dear all,
I am new here at the forum but I am an enthousiast for years in everything what is Chrysler/Jeep/Dodge. Currently I am rebuilding a 1984 Grand Wagoneer. When I bought it I drove her home and the Transmission was shifting smoothly. No issues.

Unfortunately on one day I forgot to flip back the arms of the 2 column lift. (Stupid!) I tried to drive the truck in reverse but it got stuck behind the arms. I gave a lot of throttle because I did not know there was an obstacle. Since that happened, the Transmission slips in all gears, that meant for me it was already quite worn.

So my first project was to rebuild the TF 727. I bought a rebuilt kit from Felpro and replaced clutch package, kickdownband all seals and so on. (following Youtube movie from Chrysler)
After assembly, I drove a few kms and I suspected a little delay/slippage in the Drive (D) when accelerating. I had checked before the pressure valve in the valve body and checked trottle linkage as well. It looked all fine. Few days ago I found out when selecting the first gear (position 1 on gearselector steering column) that there is no movement at all! As if it is in Neutral.

I have checked with the Handbook from Munroe, but I only can find that second clutch should be engaged and Low Reverse is engaged. I already checked the valve body but cannot find any strange things. Both bands adjusted conform spec.

My question is, what might not work well, what could be wrong? Maybe it is a small thing, so I try to avoid to build out the entire Transmission/transfercase if it is not necessary.

Any help is highly appreciated!!

best regards, Marc (Venlo, The netherlands)