Inconsistent GC-NPD results

Hello everyone,

I am analyzing organophosphates using GC-NPD instrument (6890 Series) and I am experiencing inconsistency in the obtained signals.
For my tests, I did several runs in splitless and split mode in triplicate. Each time the first injection gave the highest response and the second one lowest. The signals from the second run are 50% of the signals from the first one or less.

I do analysis with hexane as the solvent and I use DB-5 column.
I have already changed the syringe, the column (the peaks are fronting no matter what so I thought this and integration could be the culprit), the septum, and the liner (liner is not packed).
The NPD detector (Agilent) seems to give a flat and stable baseline. The bead is not completely new but with a voltage 3.15 and hydrogen 3.8 I obtain an output of around 13.9.

I am not very experienced with GC so my question is: what could be my next steps if I want to fix the issue?

Thank you in advance for all your suggestions.

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