So you're brushing your dog regularly ... but are you using the right type of brush? I have so many dog owners explain how they brush and it seems a lot of the time the wrong brush is being used and not getting deep into the coat close to the skin. The 10-20mm of coat from the skin is constantly being missed and so starts the "hidden matting". I myself pre-grooming career thought "can't see any matts therefore my Cockapoo is not matted". Wrong, so very wrong, the start of the matting hides deep in the coat!
Get yourself a pin like slicker brush and don't be shy at brushing deep into the coat going over a few times. Be careful not to go over the same areas for too long or you may irritate the skin of the dog. You can be quite "rough" when brushing so long as there are no bad tangles these brushes should tear through the start of matted hair quite freely without hurting the dog. If you don't apply enough pressure you will end up missing the hair deep in the coat where the matting starts.