different types of paint being oil and latex mixed. The oil should last alot longer, but it only sounds like the underlying coats are releasing and the top oil is going with it.
Sills and windows from water collection and condensation do have alot of water and water vapor around them. They need to be maintained often so water does not enter into the wood and undermine the paint coverings.
Alot of the time the glazing is so poor that water enters the wood from this point and works inward and causes the release via rot and mold and mildew.
Make sure the window panes create a nice slop away from the widow to glide the water off and onto the sill and off the sill to the ground. No ponding can occur.
I would first scrape off all the paint and layers and possibly the glass glazing and start off anew.
Others may disagree, but todays latex paint is superb to todays oil products. it did not use to be that way, but latex has made many great strides. I like the fast drying time and the ability to apply multiple coats of paint in the time it takes one coat of oil to dry.
So, I would go back with a good quality of exterior latex paint SOLD FROM A PAINT DEALER, like SW, Ben Moore, or KM.
Paint is a product that adheres to the trait the more you pay the better the paint.