I had real problems starting inverted motors up to about 2 years ago. Flooded or dry, and no in between that I could find.
And worse, the only advice I got was along the lines of "the secret is in the prime". Very helpful.
The situation got so bad I have an unfinished inverted motor model in bubble wrap, waiting to be finished.
Then I finally got my hands on a 2.5 cc syringe.
Now what I do is as follows:
When I get to the flying field, I burp the model inverted.
Typically the first start takes ~ 1cc before the motor will fire up, then continue burping it for a few cc of fuel, in 0.5 cc burps - after the first start it should start every time, first flick, with a 0.5 cc prime.
When I've proved that it'll start first flick I give it a final half cc prime and just turn to motor over a few times (no glow clip attached).
Then turn the model over and fuel up.
When I'm on the flight line I turn the motor over briskly for 10 or so flicks, signal the judge, connect my glow start and start the motor.
one or two flicks, works every time.