Propane Intermittent start

I bought a Toro 6000 that has the propane conversion. I love the mower but occasionally it chooses not to start. I have the fuel connector on tight enough because it backfires so is getting some fuel. Air cleaner and cleaning the spark plug makes no difference. Clearing the line and retrying makes no difference. The choke is ineffective because I am told they cut the choke cable when they made the conversion because no choke is needed on propane. It sure feels like the choke would help as sometimes it tries to start exactly like a cold gas engine but fails to keep running. All warm weather so far. Full or low tank does not seem to impact it. Eventually it has always started but twice not the same day as i wanted to mow.

Any advice would be greatly appreciated.