SRAM X4 MTB Rear Derailleur H-Limit screw maximum open, but still on 7th Cog?
First thing to check is if your rear derailleur is bent. If that is not the case, check the cable tension is not too high. Is the shifter acting funny (like hard to shift)?

See this link to see how to do adjustments: http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur