If the nipple cover was connected properly, wouldnt the nipple be destroyed, or the cover. Otherwise it would have still been connected to the air hose.
Either that or your nipple was fecked in the first place, or even the actual nipple cover, not making a perfect seal.
But, not making a perfect seal of the nipple to nipple cover would generate that affect. As for it raining and mud being at that certain place in time, well that bulls down to sods law.
God i hope it dont rain this weekend!
Hope you get it working
Darren