My apple tree hasn't fruited in three years

I have a mature apple tree in the back garden that was producing loads of fruit when we moved in five years ago. For the past three years it has flowered beautifully in spring but then barely a handful of apples appear and they usually fall off early.
Could it be a pollination issue or is something else going on? There are no other apple trees nearby that I know of. I haven't pruned it much either, not really knowing where to start with that.