Remember the 2-stroke cycle: phut, squeeze, pop, phooey.  And remember the four things an engine needs to run: air, fuel, compression, and spark (or glow).  Whatever the failure is, it's got to be one of the above.

You've got squeeze -- you felt the primary compression.

You've got 'pop' -- at least some times, 'cause you heard it.

Phooey is kind of automatically covered on a two stroke.

But do you have secondary compression, to make the fuel/air mixture "phut" into the cylinder?

(I spent a very frustrating week on this recently, with much the same symptoms, until I realized my backplate was loose on my Fox 35).

Dennis,

Tank height on your test bench might be involved. I've had very little hands-on with Drones, but seem to remember from that and photos that the engiine should have a small enough 'choke' diameter that the fuel draw should work. Still...

If the tank is too high, it can drain downhill into the crankcase and flood the engine as you try to start it. Other-side: if it's too low, the engine might not be drawing fuel to pick up running from a small prime...

If you're using a clear or translucent fuel tubing, can you see fuel being drawn to the venturii before you prime the engine?

A prime can blow through the engine at just a pop or two. The internals of the Drone's bypass and shaft porting might not have enough 'speed' to get a good mixture up top to be burnt. (A reason why blanking the boost port on some schneurle engines, or restricting the bypass on Fox 35s works - it forces faster flow since the passage is smaller...)

If you are getting a bump, I can presume plug and battery are in good shape. Frankly, I'd choke fuel in  - cover the intake opening and give it four or five flips of the prop AFTER you see fuel has reached the spraybar - and try flipping with battery on, before a stack prime. If no noise, THEN stack prime same as where you were getting that one lonely bump.

Not to worry unnecessarily about damaging the engine from a buncha juice in the case. It will stop you flipping finger,  most likely, or just burp enough to toss a slug of raw fuel up top and stop the engine. Unless it is forced through (read: electric starter) the shaft doesn't have the speed to do damage unless the engine is running at usual operating RPM.

You want to get SOME RPM... Not just one revolution.

Luck, and keep us up to date?