IIRC, the ingestion trials on the two Lightnings taken to Shoeburyness showed that the phenomenon was somewhat random. Foreign Object Damage (FOD) was a frequent and expensive occurrence to Lightning engines, and we were keen to understand more about it. Yet the trials showed that for the most part polystyrene flakes on a plate, passed under the intake of a Lightning with its engines running, stayed resolutely put! Until, that is, a random, just-right, sideways gust of wind induced the formation of a suction vortex out of the intake, and once that was established not only did the flakes disappear down the intake, but the plate too and anybody unwise enough not to let go of it...
Which is why a Binbrook starter crew, taking a short cut under the nose of a running aircraft, could occasionally find himself sucked off the ground and have a real struggle to get free while parts of his cold-weather gear disintegrated and disappeared down the hungry intake...
I lose sleep over it to this day.