Hey @Beckiejane,
I've had that problem before. I've discovered I just need to push the offending band harder into the watch until I hear a click. Sometimes, I've needed to wiggle the band back and forth a little as I push it in. Just be listening for the click.
Note: Make sure that the band is connected before any activity. Sometimes, I'll connect my band just to find out that it's not fully in. I would hate to have your band fall out at an inopportune time!
Best Answer