Can I use sealant to stick on a rear view mirror?

My rear view mirror keeps falling off. I've tried the loctite rear view mirror glue from Halfords. It worked for about 8 weeks, then the mirror came off again. I've bought a new mirror that sticks on with a suction pad (good reviews online), but this fell off within 24 hours. I was wondering whether adding some all purpose sealant to the suction pad would be successful in making it hold? Has anyone tried sealant to stick a rear view mirror? Is there any risk of the windscreen cracking as with normal glues, or is sealant better because it's flexible?