It is recommended to only use PEEK or ETFE fittings and ferrules for mobile phase connections that are immersed into the mobile phase solvents.

(This means avoid Stainless Steel, Nylon, Polypropylene etc. for mobile phase connections.)

It is also prudent to change mobile phase inlet filters often as there can be microbial growth or particulate matter that has accumulated on the filter.  These contaminants can cause pressure issues as well as adding unknowns to your analysis or noisy baselines.

Compounds from contaminated solvent inlet lines (that is, from bottle to pump), either inside the tubing (for example, microbial growth

A friendly reminder, when changing your mobile phase or filters/tubing, always do so with appropriate gloves, as bare hands can introduce contaminants as well.