Study Plan | Divergent scores in mocks

Need some help.
I have been preparing for GMAT for quite sometime and finally started giving mock.
I scored decent in 2-3 mocks.
after enough preparation I gave GMAT official mocks and scored good 740 and 760 in them.
Somehow in Kaplan and Manhattan i scored pretty bad 680 and 700 respectively.
I am demoralized again and need some serious help.
Is it predominately because I have finished all official resource and lacking some basics?
Help needed

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