I found exit mould worked better in the window frames than regular bleach because there's a soaping agent in the exit mould. Don't forget to treat/wash any blinds, curtains, net curtains at the same time, hanging them out in the sun helps with this too. I also started using old cloth nappies to give the inside of my windows a wipe over especially on cold mornings when there's a lot of condensation on the glass, and this helped as well.

ETA - also, mould often grows on dust. So if you keep your window tracks vacuumed regularly that might help as well.