Several reasons for being unable to connect to someone's hub. They may have moved it, some item of furniture may have been placed between it and you. Electrical interference from a domestic appliance may be interfering with the signal. Most likely the hubs are needing to be restarted, reset, whatever. Hubs are not the most reliable pieces of kit.
Out of curiosity what service do you have that is not installed?
If neither you nor your parents can connect in the flats but all can elsewhere, it's got to be down to the other hubs hasn't it? It would be a hell of a coincidence if a large number have gone of at once but quite feasible if just a few. I also wondered about the uninstalled service.
There's not a lot of point complaining about it. You do not have an automatic entitlement to use someone's wifi, you have the opportunity to do so if they wish to allow it. For example I am a BT customer and you most certainly do not have any right of access to my bandwidth. BT's information on the coverage merely refers to the number of hubs/accounts they have nationwide, not actually those that are enabled. I have 3 hubs in their boxes that are no doubt included in these statistics but obviously they are not transmitting.
Why not visit your mum more and use your connection there?
It wasn't clear that you meant a working BT broadband service elsewhere, which of course that entitles you to free BT Wifi. The issue clearly relates to this specific site. Have you tried removing all the wifi connections stored on your phone and restarting it? (You have to "forget" on Android, probably something more hip on iPhone.)