Hi, you should try first to inject several times the same sample. If retention time vary from one injection to another, it is possible that the system is not sufficiently equilibrated.
Then, check all possible variations : pump pressure, mobile phase pH (you should try to buffer the mobile phase), column temperature.
If none of these parameters are involved, try a new column (possible column contamination?) with a guard column.
Please FIRST provide your DETAILED HPLC method and a chromatogram for useful suggestions.
Have your teacher or the person(s) in charge of the HPLC system who have professional training assist you in obtaining the information and also, the troubleshooting needed to understand.
Check and verify the flow rate.
Verify the column has been fully equilbrated and the baseline is stable BEFORE starting an analysis.
Most importantly of all, follow good chromatography fundamentals: "Do your HPLC Methods Meet Good Chromatography Fundamentals? HPLC Training: RETAIN, SEPARATE and RESOLVE"; https://hplctips.blogspot.com/2019/12/do-your-hplc-methods-meet-basic.html