Need more torque for driving screws
drilled a 6 mm hole in the masonry, then used brown wall plugs and #8 screws


Your hole was too small and the screw was the wrong size or type. A #8 screw has a 4mm diameter.


  Brown wall plugs fit into a hole made by a 7mm drill bit and are for screw sizes 10 - 14


Plug            Hole            Screw
Colour          Size            Size
------          ----            -----
Yellow          5mm             4 - 8
Red             6mm             6 - 10
Brown           7mm             10 - 14
Blue           10mm             14 - 18


Note: the above screw sizes are traditional size or screw-gauge. If you drill metric holes, you might be buying metric screws, where the size is the major diameter of the threaded part in mm.

If you can't drive the screw using a normal hand-held screwdriver, then something is wrong and you are at risk of shearing the screw or damaging the head. Using more force is not the right answer.