I am currently encountering an issue with SDS-PAGE analysis (separating gel: 12%, stacking gel: 5%) of whey protein isolate (WPI). The protein bands are consistently appearing at lower molecular weights than expected. For example, β-lactoglobulin (theoretical MW: 18 kDa) is migrating below the 14 kDa marker, and α-lactalbumin (14 kDa) appears below the 11 kDa marker. Even lactoferrin is also appearing lower than its theoretical position. Could you please help me understand why this discrepancy is occurring?