Is the slide more jammed than it would be if the pistol was uncocked? You may have failed to recock the hammer with the subsonic round, in which case it will take as much force as cocking the pistol for the first round.
If you can account for the whereabouts of all the rounds and the gun is not loaded, you might start the disassembly process by releasing the mainspring from the back of the grip (that annoying lever mechanism). With the mainspring released, the hammer shouldn't resist the bolt movement and you'll know right away if there's an issue cycling the slide/bolt.