Normally stiffer bristles are for hot water use. Are you using hot water?
https://gardinerpolesystems.co.uk/xtreme-sill-26cm-brush-stiff.htmlLooking to order a new brush, I know this is a sill brush but is it also good on glass, looking to order a stiff brush as do a lot of 8 weeklys & I will be having many first cleans so I’d think the stiffer brush is better for getting off the hard dirt?