Can your Mum get all channels if you plug the aerial directly into the TV.
There are a number of other posts on this community where the same problem has been raised, It seems quite common with these boxes that some channels are missing if the aerial signal is either (both) too strong and too weak.
Your Mum should have got an email from BT when she signed up telling her to go to the Now TV website and create a password to finish setting up her Now TV account.
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