Because in the slot there is no restriction of a matrix the SDS-protein mycelles migrate very quickly and will be pressed to the anodal slot edge in a very high concentration.
Some samples will then plug the slot even for the buffer-ions carrying the current.
a) Very big proteins (> 2.000.000 Dalton, f.e. IGM)
b) Proteins that will polymerize when the concentration gets very high
(Collagen, Fibrinogen, Casein)
c) Tenside-mix, f.e. CHAPS and SDS
1. Reduce concentration of the samples (1/3 - 1/10), silver-stain.
2. Very slow start: first phase: 1/3 Volt and 3 x more minutes
See EDC’s SDS-Manuals: ElphoGel Kit SDS, DryGelKit SDS
3. Try also vertical electrophoresis.