Impurities can occur anywhere in the chromatogram.  Their later elution indicates they are more 'water' loving than your target molecule.  Thus, you can increase (~5 to 10%) the amount of water in your mobile phase OR use a shorter column (increase flow rate) and keep the mobile phase composition the same.

By adding more water to the mobile phase, note the relative change in retention time of the impurity to the original chromatogram.  Depending on the retention mechanism (reverse phase versus normal phase versus HILIC) and the impurities' sidegroups the peak may move in either direction.  Thus, optimize your chromatogram (API and impurities') until you are satisfied.

Impurities directly effect the retention time and peaks will be more broad ,finally, calculating of the concentration of unknown will be not accurate.

Regards