Hi nab239,
It's not clear what 'order' you're using as you're going through the major/minor concepts that the GMAT will test you on, so if you're working through concepts in an 'illogical order', then that might be part of the issue that you're facing. Review should be a big part of every Test Taker's study plan, for the exact reason that you describe - if you're not going back to review past concepts and work, then those skills might get 'rusty' (or you might lose them outright). So building some review time into your routine would be advisable.
1) How long have you been studying for the GMAT?
2) What materials have you been using?
3) What is your goal score?
4) When are you planning to take the GMAT?
GMAT assassins aren't born, they're made,
Rich