Could be Hydro Lock?
If the barrel has an oil/fuel accumulation inside, the engine will not turn over. This sometimes happens from tipping the engine during transport. It may also be the result of flooding the engine during starting. Removing the spark plug and slowly turning the engine will expel the fuel and relieve the hydro lock. Be prepared for muck to come out through the spark plug hole. Best to bung a rag over the top.
This has happened over the years on a number of different machines. Although I can not recommend that you use a blanket to (roll) your paramotor so the hole points down before turning the engine over slowly. I do and it clears out MUCH quicker when the excess can naturally flow out as well.
Not saying it IS, but it's certainly worth testing next time it stops I would say. if when you do this you do have fluid in the barrel, you can then start to look at WHY it's happening.
SW