[Rotting]

Few things to check.

First thing, stop using Wal-Mart BBs. Aside from the seam of the BBs causing jamming, the hop-up unit is a tight fit. You need to use decent BBs or you'll just shred your rubber sleeve.

Because the CyberGun/Swiss Arms SG552 isn't the best quality, the second thing I'd recommend is take off the upper receiver, leave the rest of the gun assembled. Simply dry-fire. Watch your nozzle, if it's moving properly, it's fine. If it's barely moving or not moving at all, you may need to fix it or buy a new nozzle.

If the two previous things have been remedied, then go about checking the actual hop-up itself. Because the quality of these particular brands of SG552s is not the best, the hop-up isn't the best either, particularly the bucking. The bucking in my SG552 when originally purchased was too much of a tight fit. This caused it to push down, but not come back up after re-adjusting the hop-up, and getting a ton of fucking jamming. Simply get a utility knife or disassemble a cheap disposable razorblade and just barely shave a very, very, very small amount off at one end. Put the bucking back in, push down and see if it pops back up. If it still isn't take it back out and shave just a very slight amount again. Do not overshave, otherwise it'll just turn into a piece of shit and you will need to purchase another one.

Also, it's stupid as shit, but check your barrel. I've gotten a few guns to me for repairs due to jamming and the only problem was they had shit stuck in the barrel and all I had to do was poke it out, rifle fired fine.