Can’t think of any reason for a degeneration in signal access, but if Mrs FDs phone is newer than yours, it could be that the “radio” ROM is an older version. An older ROM might cause your phone to not be as good at getting signal. You can check the version numbers in the Settings.
You can update this, but you may need to Root your phone, and faff about, although it might be worth trying to update via the HTC software first. (When you update you’ll probably loose everything, so best to make a backup, or copy stuff off first.