My bagels are flattening after I bake them. Why and how do I fix?

Agree with Nancy or you may be boiling too long

Dan Sobel - here are some troubleshooting tips. Perhaps too much rising before you bake.
https://www.homesteadingtoday.com/threads/why-do-my-bagels-come-out-flat.289181/#:~:text=Your%20bagels%20are%20going%20flat,oven%20for%20a%20few%20minutes.