Thank you for your question, As my experience in chromatography, specifically HPLC, we found overlap problem also, obviously your solvent is contaminated with a material that resolve at the same retention time of the target compound. So how to sort out this? inject your solvent only to find out that your solvent produce no peak. And see what's going on? I tell you something will help you, did you study the spectrum of the overlapped compounds? if not please do, do not depend on retention time to ensure to identify compounds. spectrum study will tell you lots of info.