The most likely issue is that you are grossly flooding the engine, particularly, getting the crankcase full of fuel before enough fuel gets in the cylinder to make it fire. When it doesn't go, you choke some more, until finally enough gets in to fire - at which point it sucks the rest of it in. Usually this just makes it weakly run, then quit, but sometimes you get enough to hydraulic lock it, or temporarily raise the compression, which will usually kick off the prop.
This is sometimes difficult to avoid on upright engine installations and at least pretty common on profiles. I haven't had the prop kick loose a very long time, and the only times were when I was burping the engine, holding the airplane inverted with the cylinder up, and intentionally putting a lot of fuel in the venturi. David's 75 used to do it now and then, I know because my back still hurts from holding the damn thing up while he puts it all back together - when it would start backwards (too much fuel), start blowing the excess fuel out of the venturi, finally getting to acceptable conditions, and then switching direction.
This is as least one reason that stunt engines are generally mounted in the inverted orientation, it's much harder to flood and generally easier to start.
This will be particularly bad when using low-nitro fuel, it has to be much closer to right to fire at all. You might consider priming it with 15 or 25%, just to get it started without having to load it up with fuel.
It's remotely possible that the compression is excessive, but that's almost certainly not true *as long as the engines have not been diddled with*. The Foxes did not come from the factory with excessive compression for 5% nitro, they would probably love 25%.
While there is some general folklore along the lines of people becoming hysterically frightened of "overcompression, OMG!!!!" for some inexplicable reason, and curing every issue with 40 head gaskets, it's almost unheard of in typical stunt engines and you will not have to wonder about the cause when you finally go over the top.
Brett