Brocock bantam 22 magazine

Could be your pellets. Or it could be the magazine just doesn't like that pellet when I got my .22 Bantam I was all prepared with every .22 caliber pellet made to spend a whole day finding that perfect pellet. Turns out as nice as the Brocock magazines are, certain pellets don't load well sometimes requiring an almost double push on the bolt. After a few double loaded pellets I found the right pellet for the gun by purchasing a Rowen Engineering single loader. That allowed me to try all the pellets without jamming. Mine shoots the JSB Exact Jumbo 15.89 best. And yes they work well in the magazine also.
 
One other thing to watch is when the gun was new the bolt was alittle stiff and sometimes if you don't give a decent tug on the bolt, you can come up just short of catching the hammer spring but the magazine cycles. Gun won't shoot so you give the bolt another pull and now you have two pellets in the barrel. After a few hundred shots the bolt on mine is now butter smooth and this is no longer an issue.
 
Sorry, I should have been clearer on what I was saying. Didn't mean that there was something wrong with the pellets themselves, just that they (JSB's) may not load well from the magazine to the barrel. I have one Bantam that shoots H&N field target trophies in 4.52 best and the JSB 8.4 clip going in from the magazine. I can single load the JSB'S but the H&N pellet is more accurate Tried the H&N 's in 4.51 and 4.53 also but not as good as the 4.52

My other Bantam in .177 shoots the JSB 10.34 best and the H&N's are all over the place. I have 2 magazines for the .22 and a total of 4 for the .177's. Change magazines and the results stay the same. The magazine on the Bantam may just be the nicest part of the whole package. 

Good luck, hope the replacement magazine fixes your problem so you can get back to enjoying a great gun

Doug B.