Eyeglass bows, or arms, are the part of the glasses going from the frame of the lenses and loop around the back of the ears. Along with the nose pads, the bows can often not fit quite right leading them to slide down the nose. If a person is wearing the glasses all the time this can become very annoying and problematic. In extreme cases the spectacles might fall off the face entirely.
Tighten the screws where the arms meet the lens frames. If this screw is loose there will be too much movement in the bows.
Run a finger between the head and the arms of the glasses to see if they are loose or tight in symmetrical places. This will give a better idea as to why the glasses are not fitting properly.
Bend the end of the arms where they loop over the ears so there is a more pronounced bend. This will help hold the glasses onto the ears more securely.
Grip the end of the arms using a pair of pliers and bend the whole arm to give a tighter or looser grip to the sides of the head.
Gently bend one arm up or down if the glasses do not sit symmetrically on the head. The ears being at slightly different levels is common in many people.