I had that problem all the time when I used the Wahl ones from P@H, even the higher end ones at £60 - I had the 3 dogs at the time (and 2 had heavy thick coats from neutering which probably didn't help) - I found they stopped clipping about half way through the second lot of clips. I think the Wahl ones can be returned for sharpening/servicing but I just kept replacing them until I bought a professional set.