Bicycle Tutor

Hi
I am having trouble with my shifter not picking up all gears with the rear derailler. It will pick up 1 2 3 3 4 5 5 6 or when I tighten the cable it picks up 3 4 5 5 6 7 7 8. The shifter is a sram attack the rear derailler is a sram x7 and the cassette is a shimano all good order could the problem be that these not be compatible.
Your thoughts Thks

Are you sure the rear derailleur is properly adjusted?
I would favor having the cable tighter than looser. If Gear #8 does not have the chain wanting to ride into the axle, then I would then check to see if the L-Limit screw could be loosened and cause the derailleur to shift the chain into the lower gears.