I'm no expert, but am going to try to answer that question!
The questions which appear in the OG and other official sources are retired
questions(from the GMAT database), but are mostly of easy and medium difficulty.
But since GMAT is an adaptive test, the questions appearing initially in the test maybe
like the ones available in the official sources, but as you get these questions right,
the questions get tougher. Hence, the drop in the accuracy.
Another reason for the drop in accuracy could be because of the timed environment,
or plain pressure of the test. Do you time yourself during practice?
As for the strategy, I would recommend you to continue the questions from official sources,
because they are your best bet. You could also purchase the GMATPrep tests and do the full tests
in timed environment, if you have not already done them.
Hope that helps you in some way!