Peak missing on new HPLC's

I have an issue with an HPLC method. As I inject my substance on old 1100 Agilent HPLC's I see the second peak, on new 1260 infinity I and II/1290 not. I use a XDB-C18 100/4.6/1.8 column with a flow of 1.1ml/min. As I put a 40cm plastic capillary before the column I see the Peak perfectly. Solvents are ACN / MeOH / 0.05% TFA. First and al other peaks have the same high, with and without the plastic capillary. As I increase the mass of the second peak substance by ten, I see also a small peak without. Colleagues have the same observations for this substance. They toke a precolumn instant the plastic capillary an see the peak again (peak high about 10 mAu). Any ideas? 

Thank you

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