Zarembad

Hello New to the forum, and to hot tub ownership. We bought a new hot tub, and have followed the recommended water maintenance to a TEE outlined by our dealer. We are using bromine, and currently have a floater full of tabs. We are using test strips, and have gotten all levels perfect except the bromine. The bromine is consistently low. Opening the floater seems to do nothing. I figured we had not built a bromine bank when we started, so I continued to shock it until the bromine level went up to 15-20 and held for a day. Once the bromine went under 10, (2) kids used the tub for 30 min. The bromine instantly dropped to 0-1 again? What am i doing wrong. I have read all the posts i can find, and nothing seems to help. Thank you in advance for your help