Having to think about this one because I run into all sorts of problems, depends on the track. Sometimes I can't get the production right, sometimes it might be performance issues... sometimes it'll be getting stuck on a track and not knowing where to take it. For me, its probably a mix that becomes too convoluted. Sometimes i'll just end up doing a track thats just... too much of a good thing I guess, I'll want it to be really long, really epic, and i'll have loads and loads of ideas... and I just can't translate it quick enough. By the time i'm half way through, its not finished, and i've lost that spark for it and i'm stuck with half a great track - I KNOW where I want to take it but I just don't want to be arsed with it, so if I do carry on, it'll be forced and rubbish etc etc.