Has anybody got any sugestions or advice on keeping knots out of a cats
fur?

I have had cats for many years, at present I have three cats, two of which
are semi-long haired, although I brush them several times a day knots keep
forming, I am unable to gently brush the knots out as they are more like a
matt and they start right next to the skin. The same thing happened last
year when they got their winter coat.  Once they molt and get their summer
coat the fur does not knot or matt but when they get their winter coat, it
starts to knot again, and no matter how much a brush them the knot/matts
keep coming, you cannot see the knots, but I can feel them when I'm
stroking the cats.  My friend has two semi-long haired cats, same age and
very similar to my cats, but her cats dont seem to get knots/matts like
mine.

Does anybody have a similar problem with their cats?  Any advice would be
appreciated.