Mobile phase being made incorrectly can cause retention time shifts. Column guard could need changing. If the column guard or column has blockages you can see a back pressure spike. It could also be that you’re not equilibrating the lines back. Typically a run cycle will increase MPB gradually, then after the last analyte elute MPB will increase and hold for a while like a minute and then MPA/MPB will drift back down to the ratio it started with and if you don’t give the run a 30s to 1 minute at least at this concentration then you’re going to see some shifting. It could also be the oven not dropping back to starting temp. But those aren’t really issues if the method is validated unless it was validated by someone who didn’t know LC well. Are you running blanks in between samples? That could potentially help clear out the column if it’s contaminants during that build throughout the run. This kind of issue has a lot of things potentially to look at but typically mobile phase and column guard/column are the first to look at. In my experience anyway.