Do the leaves fully develop before they are lost? What do the leaves look like when they fall off (Yellow splotchy, green, etc.)?

They look fully developed to me, yes. They turn yellow and shrivel and die. The one that just lost three leaves and only has one left has a couple new leaves starting to grow, but this has been going on for a year. Thanks for asking.

The most likely easily remedied culprit is spider mites which can be treated with a 3 in 1 type product or Neem oil. So let's start there.

There could be other reasons such as too wet/dry, not enough sun, fertilizer burn, etc.