Low Reproducibility in vitamins peaks


I am trying do develop/validate a test method for water soluble vitamins (B group ) and I am facing very low reproducibility. 

I am using a 1200 system and my current mobile phases are 25 mM NH4H2PO4 and methanol (gradient program).

The problem is the method works perfectly fine the first 15-20 injections and then some of the vitamins start to co-elute and show some tailing. Even after column washing. 

I am using an Agilent HC-C18 Column 150 x 4.6 mm, 5 um.

Please help

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