Fan keeps Burning out

Hi there can anyone help ive had a new fan installed in the bathroom and they keep burning out or breaking down this is the 3rd one in as many months - does anyone know why this could be happening?? Any help much appreciated.

Duct work blocked - seens bee's nests in them before!

If not, is rain blowing in? The louvred outside covers that rattle in the wind are not my favourite, you can get a weather cover cowl like a lean-to roof covering a flap (I live in an exposed coastal region) which I find better (you may find them in the tumble drier section).

Like this from Screwfix (search "Vent Cowl")