I replaced a blackout blind on my Velux window, but the grey plastic securing brackets had also to be replaced. The new ones came with the new blind. I cannot get the blackout blind cartridge to click into position. It just drops back out. As far as I can see, there is only one approach when bringing the blind cartridge towards the brackets, straight up towards the Velux window, so that the blind's black side fittings slide over the grey brackets - but no click!