Lack of Linear Response

I operate an 8355 SCD and am currently running some recovery tests for my standards. When I first ran standards when the instrument was relatively new, I got nice linear response for almost all of the compounds. I am running them again and I am getting a lack of linear response for certain compounds. I am getting decent linear response for s-methyl thiohexanoate, wildly excessive response for dimethyl trisulfide, and non linear response for 3-mercaptohexanol. 

Any ideas as to what may be causing this? 

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