Sounds like the props are tightening down due to how the direction the shaft rotation and the propeller and motor shaft threads interact with each other. (I'm no guru here, especially for the set-up you describe - just voicing a possible solution).
In the metal-crafting world it is sometimes necessary to put a solid round shaft into a solid ring which has a slightly smaller inside diameter hole in it to form what's called an "interference fit" (the shaft is meant to be in there very tight, but not rotate inside the ring when in use). To do this the shaft is dipped in liquid oxygen or liquid nitrogen to shrink its outside diameter sufficiently to insert it into the hole. The two are then allowed to stabilize back to room temperature and, wha-whooo, they are tightly joined.
You could try reversing this process with your props and their associated motor shafts. Metal has a higher coefficient of temperature change than does carbon, so what I suggest is possibly rapidly cooling the tip of the rotor shaft with something like liquid oxygen or liquid nitrogen, then using whatever prop removal tool came with your kit to remove the prop normally.
Where do you get such an item that could cool the prop shaft tip like that? Try a drug store! Look for a wart remover kit that has a small canister of pressurized air in it and a small fibrous applicator tip that attaches to the can. Set that up and then try discharging some of the contents of the can while holding that tip against the end of the rotor shaft for a few seconds.
Who knows... it may work!