Varying peak and retention time in GC-FID Agilent 8890

I am using Agilent 8890. I am trying to set the standard for my GC-FID, I am injecting 5mL of methane gas standard with a gas-tight syringe through a switching valve which is set up to the left of my GC. However, I am getting different peaks and areas at different times for the same concentration (1000 ppm) of methane each time. I am using the GS-Carbonplot column. What could be the issue?

Any advice will be really appreciated. Thank you.

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