Maybe magnetism or an oil fouled hairspring. Try degaussing the watch and observe hairspring while the balance is beating to see if any of the coils are sticking together.
It should work fine if you use it properly- place the watch on the demagnetizer, depress the button, and then lift the watch away while continuing to hold the button down.
the hairspring spring coil is stuck together, either from dirtiness, dried oil or they're magnetized
if it was recently dropped or hit against a hard surface, e.g side of door or table while wearing. this will have caused something to be dislodged or misaligned