If migration time shifting is the only problem you are observing from your electropherogram, it is recommended that you check the accuracy of your power supply. If you find that the power is clean and constant then you should check your capillary.
Sometimes the inner surface of the capillary becomes modified by the buffers, solutes and the EOF. It is not easy to restore the capillary to your original state so you should change it. Make sure that the capillary is conditioned properly by following the conditioning procedure of the manufacturer.
Once you have installed the new capillary, we recommend that you run a set of “Standards” including standard “conditions”. This will either prove that you have a suitable system and are ready to run your electrophoresis or that you have a problem in your instrument.