RT and pressure variation

I am having 1200 HPLC system with AIV, with isocratic elution one sequence has performed with azithromycin content, in result set injection of 5 replicate standards are ok, after starting of sample injections observed pressure fluctuation run by run and at the last bracketing injection, pressure and RT has observed within limit with no any fluctuation. At same condition performed leak test and pressure test found passed, checked MCGV found no leakage. what can be the cause??

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