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It's impossible to answer this without a proper examination of the passport, but the usual reason is technique. There now several sorts of passport gates in the UK, and it is the case that some of them seem easier to use than others. In other words the gates are more or less tolerant of the different ways people place and hold their passports down. If the chip was brokern then clearly that would have been advised to you. The paper page is actually slightly more reliable than the previous laminated version. The best I can suggest is that you ensure the entire passport is kept flat on the patten, with the minimum pressure need to keep it flat, don't move the passport at all on the patten and remove any cards, leaflets and other such items that would interfere or reduce the NFC transmission from the passport. If your phone or watch have NFC items attached (e.g. credit cards, travelcards) keep them well away. from the passport machine.
In extremis I guess you could simply apply for a new passport.