I think he said it ran for 3.5 hours after bottle #1 went empty. I'm wondering if bottle #2 wasn't really full and he's now out of propane in both bottles. Only way to know for sure is to weigh them.
Could be a regulator problem if you have an auto change over regulator. Might want to physically swap the bottles out so you are using the side of the regulator and the hose that worked before bottle #1 went dry.
Also could be a sail switch as someone else suggested. Igniter won't fire until the sail switch has closed for a preset amount of time.