disappearing compounds


We have lately an issue with one of our LCMS, during our multi-residue analysis of pesticides method, some of our late eluting compounds started disappearing (or just the response dropping 10 to 1000 times). They might be gone for a certain amount of time during the day and reappearing in the late afternoon, or after several days, and then disappearing once again. We already tried running liquid solvents ad rewrite the method, but we still couldn't fix it. Does anyone has any suggestion why it keeps happening (in only one machine) and how could we fix it. 

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