Same issue, and got a replacement from Fitbit, it broke within 3 weeks and got sent another one, and again that broke after a few weeks. It is a clear design fault and one they never seem to want to fix. I got sick of never wearing it because of broken straps and replacements taking a week or two to arrive I bought a strap from a third party that had the part that kept breaking made in metal rather than plastic, which worked really well. Eventually, I ended up getting an Apple Watch and they have a much better system to hold the strap on.