After getting a new kitchen a number of years ago, we had the same problem particularly when boiling potatoes for example. Builder suggested that the pipe going through the wall was too large or too small, for a tight fit to the back of the extractor, but it turned out that a poly sheet in the filter compartment hadn't been removed. All was ok once it had been. That said, given the heat given off from the gas hob, I was still surprised to see condensation on the back wall. You could put the extractor on full power, put a pan of boiling water underneath and check outside to see that the extractor is working
Without seeing the set-up, it could just be the same as ours..
All works, connected ok etc, but simply does not have enough ooomph to extract quickly enough. Basically, we could have bought a better one, but bought a basic cheapy.
I think in ours, the filters cause too much restriction to the air flow. When you remove the filters, it's fine.