First, check if there is anything preventing your zoom button from moving from side to side. There might be dirt or residue preventing it from functioning properly. If there is some sort of blockage, take a toothpick and carefully scrape away at the dirt or residue surrounding the button. This should remove most of it, but just to be sure, take a cotton swab with a solvent on it, such as rubbing alcohol, and wipe away the area surrounding the zoom button. This should solve your problem.
If there is still an issue, your zoom button may need to be replaced.