Whenever I knit a test gauge, my row size always comes out exactly as the gauge states, but my stitches gauge is always too short–can anyone tell me why this would be? Also, what could I do to correct this if I’m already using the exact yarn specified in the pattern? Should I try a slightly bigger needle? Help! 
Gauge problem
Stitch gauge is the critical one. You can always adjust for any row gauge difference by knitting for length rather than number of rows.
If you have too few stitches per inch, you need to go down a needle size.
If you have too many stitches per inc, you need to go up a needle size.
In my opinion, stitch gauge is more crucial (I don’t want to say more important because row gauge is important too) than row gauge so I always go with the needle size that gives me the correct stitches/inch. If you are making a sweater, for example, even a small deviation from the stitches per inch can make your sweater much too tight or much too baggy. However long or short the sweater is can be adjusted much easier than how wide it ends up being. Also, patterns will often ask you to work for a certain # of inches, vs. a certain number of rows, so to me that means that there can be more leeway in the row gauge and you can still get away with it because you are knitting to the same LENGTH, not the same number of rows.
Hope this has been helpful.