The internal dielectric constant of hydrophobic core is default 1 in MMPBSA. But actually the internal dielectric constant of hydrophobic core is greater than 1. You should try with increase the dielectric constant.
"On the Dielectric “Constant” of Proteins: Smooth Dielectric Function for Macromolecular Modeling and Its Implementation in DelPhi" you can read this article for dielectric constants of proteins. But on an average 4-6 value is good.