While the OP's issue seems to have been resolved other Q100 / Q120 owners should be aware that for a time the regulator used on this Q series of Weber grills failed at an alarming rate, displaying the same symptoms the OP described. I replaced the regulator on my own Q100 twice under warranty, again on my current Q120 also under warranty, even recently replaced the regulator on my Q220 twice now even though it's a different model regulator. I suspect these regulators were all manufactured in the same Chinese factory and all (for a time) suffered the same failure rate.
Any Weber owner should also note that the tank > grill adapter hose sold by Weber has a noticeably larger opening on the 1"-20 threaded fitting that screws into the grill's regulator than do third party hoses that otherwise "look" the same ... so if gas flow is an issue with your Weber that too could also be a reason.