Have you cleaned it thoroughly? Does the same thing still happen after it has been cleaned?
It is possible that the cylinder will close more easily if you press the cylinder release until the cylinder has been fully seated. Still, this is not a good thing.
It is possible that your pistol's cylinder suffers from end-shake. That is, the cylinder is somewhat free to move fore and aft on its arbor, even though it is fully seated.
If the cylinder has end-shake, each cartridge will set back as it is fired, and will press against the recoil shield. This will cause enough friction that trigger-cocking (firing DA) becomes difficult.
End-shake could also cause the closing cylinder to hang-up on the recoil shield, as you try to close it.
Fully close the empty cylinder, but do not cock the hammer. Now, try to move the cylinder forward and backward within its mortise. If it moves noticeably, it suffers from end-shake.
The cure is to place a shim on the cylinder arbor, ahead of the cylinder. This is a job for a good gunsmith.
(Your gun may have been put on sale by its previous owner because of end-shake.)