Third mini box issue

[JANE+TRASK]

Hiya. I've a 2tb main Q box and two minis. My house is old and odd shaped so I also have 2 sky booster boxes. All worked OK until I added a third mini last week. We never watch 3 minis and the main box at once, only ever main box and 2 minis. My problem is that one of the original mini boxes now keeps losing connection to the main box. It's always the same culprit! How can I avoid having to reset this one every day? Thanks for any help.