This has happened to me in the past, actually a few times. I agree this is more than likely to happen with ashes in the fire pot than not. I also agree with all said here, however, if getting ready for a cook, with no time to clean the pit before hand, I simply turn it off, and restart it. It always lit on the second try, once it took two tries.
I also agree that this could be the start of an igniter failure. If it is under warranty reach out to GMG for a replacement, if not, they are cheap enough to purchase.
Good Luck with it.
Bill