Optical tweezers that do not work properly.

Fioda

Hi!
I have the following problem: the optical tweezers attract beads but when the beads get to the trap, they are repelled. Furthermore, when once in a while a bead gets trapped, it goes out of focus.
The tweezers are coupled to an inverted optical microscope. They are made with an IR laser and an aquous immersion 100X objective (NA 1)
Some trick of what to do?