Sticky pads are not suitable for all mirrors, only the ones with a relatively large flat base. Some types have a much smaller metal mounting point (which is separate from the mirror) that has to be bonded to the windscreen using a special adhesive.
If it is supposed to use the sticky pad, then get both the mirror and the screen as clean as possible, using a solvent cleaner such as IPA, or even celluslose thinners. Then warm up both the mirror and the screen using a hair dryer (don't go mad on the screen otherwise you could potentialy crack it, just gently warm up an area around the mounting location. Obviously don't touch any of the freshly cleaned surfaces with your fingers after cleaning them. Correctly fitted the sticky pads work fine, but if fitted to a cold/dirty screen they will fall off very quickly.