I live in a household with BT Halo which entitles me to double data each month. My mobile plan is on a different account and after the initial set up my data was doubled up each month and continued for several months. But on the last phew occasions I've had to link my account again to get the benefit. Last month I attempted it again but on this instance the link wasn't there.
I contacted BT support who eventually resolved the issue which I was told that it had been caused by an issue with the back end system that had now been corrected. And was assured that the benefit would roll over each month. My allowance was reset on the 21st of September and the issue persist.