either the foot valve is bad or you have a hole in a pipe between the foot valve and your house. I would go with a submersible pump, but that is a lot of money. Foot valves are cheap so the mark up is very little pumps are a lot more money so the mark up is a lot more money.
submersible pumps are nice because you don't need to prime them.
Check your well-troll tank to make sure it is ok. You can knock on it to see if it sounds hollow on the upper half. That would be good the lower half would thump.