If I were you I would break out the jewelers eye. Spider mites or fungus gnats it's hard to tell in the pictures. One female gnat will lay up to 300 eggs that larvae will feed on your roots, the mites will suck the life from the plant from the underside of the leaf. Either way that would be the 1st thing I look for. Act fast these could be deadly if that's the problem.

Good Luck!