Increasing shift in retention time

Hello,

I have GC-FID 8890 and there's problem is suddenly appeared, the increasing shift in retention time for concentrated samples and for some of diluted samples that I previously injected and the retention time didn't have any shift.

Please consider, I don't change any parameters of the method such as flow gas or temperature.

I did some procedures but the problem is still persist : 

1- I replaced septum 

2- I perform inlet and septum leak test and obtained pass the tests 

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