I was having trouble with my F40 RS fixtures in the basement not always starting due to the summertime humidity (and yes, they're grounded). My brother suggested a cool idea: wrap the lamps with grounded wires. I took some small strands of copper wire and did just that, establishing a ground by wrapping the end of each wire around a screw on a Romex clamp on each fixture. Now the fixtures start perfectly. I may try this on two of the lamps in one of my garage lights, as they sometimes won't light after a rainstorm.

If you don't mind messing up with the wire, it could work.
Otherwise try to wash the tubes (only the glass tube, not the socket assemblies) using a cloth and some detergent. Usually it is not just the water, but water soaked into the dust sticking on the surface, what cause these problems.

And grounding the metal caps on the lamp ends (so ensuring the wet mess is always on a ground potential) could help as well, while it would be less obtrusive.

Or convert to a preheat, the starter offers way sufficient voltage even for a wet and dirty lamps... It could be just a starter connected with each terminal to one contact on each end: The filament heating woud be provided by the ballast, but the high voltage kicks by the starter...

I see the same issue in one of my T-12 equipped church buildings (Most of the ballasts are 1973 vintage) after a rainstorm.  Thing is though, we have low humidity up here in Colorado.  Hardly anything over 40-50% in the wet season.  Not all fixtures do this, but many of the 2 lamp F40T12s do.  If I flip the switch a few times, they usually will fire up then.  I am regretfully upgrading this building to T-8 later this year.   

Hey, at least you'll get some ballasts and lamps out of it, right?

If they're cheap LPF ballasts, 34 watt lamps might also start better.