I have read somewhere that it is the presence of iodine in the water that governs the rate that the Tadpoles mature. Mine usually do mature but sometimes it is getting towards autumn before they try to leave the pond.
I spent a little while on Tadpole watch today in my wildlife pond. I now have a full range of development stages. Some have no legs, others have two and few have four. I didn't see any ready to leave the pond as they all still had a long tail.