Marilyn,
Have you ever had creeping ivey growing near or around the frames at any time??
If you have, then the little buggers have suckers that create what looks like fly poo, you can use all sorts on this and they tend to come back after a few day's.
If this is the case the only way to get rid of anything on upvc is by useing this method and that is by useing cellulose thinners, apply this to a cloth wipe over the cill, but instantly wipe over the cill again with a clean dry cloth, do not leave these thinners on until dry or your cills will melt.
This will get rid of anything but use it with caution.