MTB 9 speed sram shifters/rear deralier, shimano 9 speed cassette (HG?) only shifts to 8 cogs I have an old KHS Team ST frame that I just threw components on about 15 years ago.  the drive train is SRAM esp 9.0 shifters and rear derailleur with a shimano HG cassette.  I have never been able to shift into the smallest cog.  I wonder if this frame (most likely 1999 or 2000) is compatible with a 9 speed drivetrain or if I am just absolutely horrible at bicycle maintenance.  I am also no longer certain that my chain is a 9 speed chain, but it probably needs to be replaced so I will be doing that anyway.

What are some things I can look at to fix this problem.
If you've already checked the lower limit screw of the derailleur, and also checked for a bent derailleur hanger, another thing to consider is cable friction. To get to the smallest cog, the derailleur spring has to move the derailleur all the way to the outside. While it's doing that, the spring is pulling the shifter cable out of its housing.  If there is a lot of dirt/oil/gunk buildup inside the cable housing, that would make it difficult.  

Given that the bike is fairly old, it's worth removing the shifter cable and lubricating the inside of the cable housing. Then you could either reuse the old cable if it's in good shape, or better yet, get a new shifter cable (should cost just a few dollars).