The problem is that your under layment or felt paper more than likely does not have the ability to let the moisture rising up from your appliances taking showers etc...the felt traps the moisture trying to escape so it stays in the attic and due to the humidity the moisture attaches to the nails just like a coke can you take out of the refrigerator and accumulates until dripping down...bigger problem is any moisture that does pass through the nails gets trapped above it because its trapped by the non breathable felt and it fans out on top of the plywood decking under the felt almost like a trash bag on a wrestler tryin to lose weight quickly..they are called cats eyes if there is rot around the nail head inside its way worse on the top side...change your roof and get a system that will address that and increase your ventilation as well