People can learn to swim at any age. So if you child hasn't learned by 7,9, 13, 22 or 40, it can still happen.
How much time do you spend at the pool? How much time do you spend in the water with your kids?
6 weeks of lessons a year, is probably not enough to learn in 2 or 3 summers. Especially if you're not getting in the water for play time in addition. Give it some more time.