your loads are not generating sufficient pressure. This makes it likely that:
1. Your loads are actually too light; or
2. Your bullets are not properly crimped into the case.
There is also the possibility that your gun was shipped with too strong a recoil spring, but I suggest you start with the more obvious causes.
Compare YOUR loads with the data here:
http://home.columbus.rr.com/jmaass/ipscload.htm
You might want to bookmark that site!