Example : This beats me!.
Got me--I don't know the answer!.
A vexing problem.
This question really stuck me.

Synonyms : amaze, baffle, beat, bewilder, dumbfound, flummox, get, gravel, mystify, nonplus, pose, puzzle, stick, stupefy, vex

Example : There was a new development that complicated the matter.

Synonyms : complicate