Atwood water heater ignition issue

Thanks Chris and Doug. You steered me in the right direction. I believe the problem is with the electrode. I took a closer look at it and it had a greenish scale on the metal tip. After cleaning it with emery cloth it fires first time every time now. The ceramic shield looks fine but there is some "caulk" on the hot side where the electrode exits from the ceramic which is crumbling off. I think I will order a new electrode and try to avoid future problems with it.
Thanks!