When the loader does not drop, try putting the lever in float and see if it will drop.
If it drops, that means that the QD is good.
If the 3pt lever was causing backpressure, it could cause the loader to stall, and at that point, recheck the 3pt lever.
A 3000 psi hyd gauge will tell you if there is pressure in the flow path, and is a good indicator that something is not right. Should see very little pressure with all valves in neutral.
When using the loader or any cyl and d you see relief pressure on the gauge,and nothing is moving, that is indicative of a blocked QD.