Method development

Hi sir,
I have a doubt regarding the method development.
Actually I am facing a problem in assay method development, when we start the sequence with new column it’s coming good but after first sequence my component peak merging with blank peaks and whatever programme we change we are getting the same pattern, same story we are facing daily, our buffer concentration is 0.06M of Phosphate buffer which includes disodium hydrogen phosphate and potassium dihydrogen phosphate.
Please give me some clarification about this why is happening.
Thanks& Regards