My US passport works fine on the US machines but not at the Heathrow eGates. Still the agents standing by the eGates make me go to at least 2 machines every time before sending me to the boarder control officers. I strongly believe that the problem is finicky passport readers or some internal checks they have implemented in the software. To make things worse the machines give no meaningful info, just "seek assistance". There is no damage to my passport and no cover. The entire process is very unrefined.