Ronnie: Sometimes you need a good scrub with some powerful detergent to get through the ingrained muck.
Try squirting some detergent direct onto a wet brush, or dip your brush in a bucket of strong detergent solution then scrub the glass as best you can. Let it soak for a while, then clean as normal, giving it a good rinse.
While doing the detergent scrub, don't have any water flowing, or at least only enough to get a good foam on the glass. Afterwards it's vitally important to give it a thorough rinse to wash away any detergent.