Tuning Xisico XSP120D/Beeman P17 won’t fire

Hi. I got a couple of Xisico XSP120D from flying dragon, one was supposed to be good the other not. Well, I shot both for a while with the only difference being one had a heavier trigger. 

Today during an informal 10m match, the bad one started to randomly not fire. I could feel the pump resist, safety off, click, and nothing. After 6-7 attempts it fired, then same thing. The trigger seemed to be hanging up some, so I thought I might try to fix it.

I researched and found an article from BB Pelletier. I took out the pump, seal seemed ok. I took off the grip, partly disassembled the trigger, looked ok, didn’t take out the hammer, put everything back, and it worked great with a better trigger, it shot hard for 5 shots. 

Now it pumps, safety off, pull the trigger, click, and the air “breathes” out, like “pssss”. It must make a lot of pressure because it takes a second to all come out. I did make sure the barrel was seated where I thought it would seal good with the seal where the air comes out (breach seal?)

Before I research further, could maybe someone more knowledgeable, point me in the right direction? I’d really appreciate any help. I do like how easy it is to take apart and work on, and I marvel that for the price the Chinese managed to make a copy of something with such close tolerances. 
 
I'm sure you already checked this but mine does that when the barrel is plugged. Also check the Piston o-rig, Might be starting to to go. There are a lot of you tube videos about rebuilding these. They are very easy guns to work on. MIGHT want to check your pellet seating as well. If the pellets is not seated properly it will gouge the breech seal causing that strange psss. I know all pretty obvious solutions but I really can't think of what else could go wrong. OH also check that the piston isn't all gummed up with chinesium grease. Mine was HORRIBLE and took a lot of cleaning.
 
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Stefan was right. Fixed, I ordered a set of o rings, changed the piston o ring, it looked fine, and it's been shooting for days. Plus was, it had a gritty trigger compared to the other one I got, taking apart its trigger assembly trying to diagnose, putting it back together the trigger was nearly as good as the other one, don't know why everything looked good. I'm not so sure I repaired it, as much as I got lucky, but y tube videos helped me try.
 
Thanks for all the info in this thread. Just today, I had the same problem with one of my Beeman P17s, everything was working well... until I *forced* a Crosman Piranha pellet into the breech. The head size was simply too large and caused the barrel to essentially be plugged. So, I did a search and ended up here.

I took a pointed wooden dowel and pushed the Piranha past the breech and all is well.

Thanks again!