It is not totally out of the realm of possibility that it is leaking too much compression. I purchased a GS .40 a year ago and it had weak compression and would just pop a little. It had just enough compression that I questioned whether it was operator error, or if there was something wrong. I ended up sending it back, and it was replaced with a new engine with MUCH better compression right out of the box. Ringed engines do need more break-in that ABC, but they should have enough compression to start up, so you CAN break them in the rest of the way. After a couple tanks of fuel, most of the leak should be gone and it should have "decent" compression.
Ross