Have had this problem before. Check your RPM sensor and make sure it is plugged in correctly. On mine the plug was faulty and one of the pins pushed out and there was no connection.. Check and watch the screen when you start up. You may not be seeing any or erroneous RPMS, this will casue the propane selnoid not to open.
The second thing, if this is not the case, to check and make sure you have not frozen up the propane selenoid. Empty you tank and then refill it, let it sit for a few mintues. Try again.
The biggest problem I have is knocking my temp probe out and getting a failed start.
These engines run great and put out a trmendous amount of power. They require a little more attention during the startup, but I love mine (both of 'em).
Good luck
Tommy