A couple things to check. Both will need removing the inside door panel and window molding.
Check the window channel, the felt “u” shaped pieces on both sides. Make sure it is secured at the bottom and all along the travel path.
On the glass frame bottom there is a about 5/8” wide and 3/8” deep channel that two round button shaped heads ride in. The buttons are attached to the window regulator arms. The buttons tend to wear to a flat spot, which if worn enough will slip out of the channel. If that is the case post as such and a possible repair process option, short of replacement of the whole assembly will be offered.
