Many thanks for your helpful comments.
I had an inkling that would be the case.

The problem with any hover mower is that the very fact that it needs a down
draught to hover goes against the suction needed to pick up the cuttings.
This is overcome by using the down draught to blow the cutting up into the
box. Keeping the box vents clean along with the hover vents always improves
performance, but requires constant attention. Almost any wheeled machine
will pick up better, be it electric or IC powered, just go for the best
quality machine you can afford.

Mike