Generally speaking, your beer - or sometimes cider - on tap will fob (ie, foam too badly to be served) if the booze is too warm, if the glass is filthy, if your lines arent cleaned correctly, if theres too much gas on, or sometimes, if the wrong nozzles been fitted. And then sometimes its none of those things, and still theres more froth than there was in the speeches at the last Labour Party conference. Youve had the lads out to troubleshoot the cellar and the lines, and theres nothing wrong yet the bubbles still come. This is what comes of building pubs on ancient Indian burial grounds.