What do you mean by the sample is faster than the ladder ? Have you stained your gel already or do you mean that the migration front moves faster ? A picture would help.

Often, the migration isn't homogenous throughout all of the wells, resulting in a "smiling" shape (the middle migrates faster that the sides). If you are used to put your ladder in lane #1 and your samples in the middle, try doing the opposite (ladder in the middle).

Also, make sure your gel has a similar composition as your running buffer (tris-glycine, Bis-tris, MES, MOPS etc.)

Are you using Discontinuous SDS-PAGE? What's the T% of stacking and resolving gel in your SDS-PAGE?

SDS-PAGE is an electrophoresis method that allows protein separation by mass

Štěpán Marhons

The major casein subunits may be distinguished by electrophoresis and are designated as a ( 22-23.7 kDa ) , b (25 kDa), g(24 kDa), and k-caseins (19 kDa)

I prefer 15% PAGE for such protein as casein because usually in 10% PAGE you can see only 25-250 kDa

What do you mean with it is migrating faster? Do you have any photos of your migration? What marker are you using and what bands are you expecting to see for your samples?

Example 15% SDS-PAGE of non-fat milk containing casein

Standard: thermo cat № 26630 https://www.thermofisher.com/order/catalog/product/26630#/26630

Loading per well: from 2 µg

And example from inernet -casein