There used to be a saying, "The softer the wood, the harder the knot". I'm not sure if it's always true but knots can certainly be like bits of concrete.
I onced watched Mr Morita, the Japanese tool demonstrator who used to attend the Axminster Woodworking show, preparing the softwood for one of his amazing constructions. He very carefully pared down each knot just a little with a chisel before he started planing. He used the chisel bevel down of course. He knew just how much to pare away so that the whole surface would be flush after a few passes with his wooden smoothing plane. Which, although it looked like a battered old piece of scrap oak, would take shavings so fine they were translucent.
I've tried doing this "spot treatment" myself with pine and it certainly helps a lot.