What could be the problem of my truck not cranking over?

If it rains and I can park the truck where the front is downhill and it will not crank. the motor won't even turn over. But if I back it in to where the rear of it is downhill it does fine. Trying to get on idea of what could be going wrong. I had a computer diagnostic done last week and the only thing it showed was a bad connection to my starter. I got it repaired. Keep in mind the only time it does this, IS WHEN IT RAINS.