Thank you for all the replies. By 'impossible to sharpen' I meant impossible to keep an edge that will do any more than polish the wood I'm trying to work. I've tried both stone and my Stihl chainsaw file to sharpen it - I sharpen my other tools (knives, axes, lawnmower blades, chainsaw) and they keep their edge for a good time. I agree that some things shouldn't be rushed but I also find that a regularly used blade usually just needs a few strokes to bring it back up to properly sharp.
It is an old blade that was bought off the auction site, and it has turned into the main reason I'm not making progress with bows as it's just too frustrating trying to use it when everyone else seems to be able to turn half a log into a rough bow in a couple of hours!