Undermount sink caulk failing

I have replaced the caulk for the kitchen sink twice in the past three years. It is leaking again. Both times I made sure it was cleaned with alcohol and dried it, and turned off the water so nobody could use the sink for at least a few days after putting new silicone caulk in place. The last time was just one year ago. It's a pain and I don't think it is supposed to be a yearly chore. The sink seems to be installed well, it doesn't move at all. The sink is stainless steel and the counter is granite.

Am I messing it up by letting my kids (30 and 20 pounds) have sink baths? Usually one at a time but it's a double sink so sometimes one in each sink. Is that just too much weight? Or is there something else I need to check for and remedy before recaulking?

Thanks!