There are hydraulic actuated "gear down" locking devices located below the seat deck for each main gear leg.  I would remove the front seats and look under the floor to see what that item is doing. There is pleanty of pressure to move that gear against the airstream. That is where I would start - going from your description of the problem.  (the actuators are located toward the rear of the front cabin - on each side)
jack.

Yea, there is really no way to troubleshoot this without doing some specific checks with a multimeter..  There are some switches in the downlock curcuits that I would want to check, also the pressure switch control on the powerpack. I would, for sure, put the aircraft on jacks and see what the emergency hand pump does with this situation.  It could be, and very unlikely, that the system is low on fluid.  The powerpacks are really well made so unless it has a bunch of cycles, I wouldn't suspect it until everything else was checked out that has an effect on its operation..

While on the jacks - can you hand pump the gear down and into the locked position?  This would be another hydraulic pressure source to try. The hand pump emergency extension system it is designed to provide enough pressure to lock the gear down and give you the down and locked gear indication light.  if this turns out to work ok, then you can probably assume that the powerpack is not providing enough pressure - and go from there...good luck and be careful with that airplane up on jacks..

That could be expected...I suppose it was removed for that housing (crack) inspection  AD that was later resinded..what a boondoggle that was..