There was another thread on this topic a couple of days ago, only it mentioned Attitude polish rather than Sinful. Various reasons were given - old polish, try and use some thinner, keep bottles in the fridge if you can etc. Leighton Denny was on QVC the other day and he said that bubbling appearing in polish on nails was as a result of the coats of polish being too thick. Don't know if this will help you - could you try thinning the polish?