Just a datapoint - May be useful to some or not - But I had failures for about 10 times, having read someone suggesting I should push in firmly and down hard on my passport when using E-gates.
Instead, the last three times I've done the opposite, resting my passport relatively gently and not necessarily pushing it all the way in and been admitted all three times. Once, I did have to insert it slightly further, but it worked the second time.
Worth considering perhaps!