fwiw - I had similar on my 643.
Bought it from my BIL who said bucket would not tilt forward. Scoop was OK, Up & Down OK.
The spool was hung up in the "bonnet" on the back of the control valve.
Linkage appeared to move, but not enough to get the fluid moving.
Your lift spool should be the top one, Tilt / Scoop = center, Aux Hyd = bottom.
After getting the bonnet bolts loose, the Tilt / scoop would not let the aluminum bonnet come off.
Took the pins out of the front of the spool. ( My - what a treat! ) and soaked the whole mess in some solvent.
Had to drive the bonnet off the blind end of the spool and it was all corroded inside the bonnet.
New seals, cleaned up bonnet, order a new bonnet from BC parts, back together and working.
Waiting for a chance to need to swap that bonnet.
BTW - I did not remove the valve body from the machine. Did it from the back, which probably made it harder, but I did not have the wrenches needed to remove the lines from the valve.
Hope you find the reason on yours!