Mismatch between experimental and modelled data - forcefields?

[Xaiver]

Trying to model an isotherm for Ni-dobdc between 0 and 1bar with H2. Getting an isotherm which underestimates adsorption at low pressures and overestimates adsorption at high pressures when compared to experimental data. Is this likely due to the forcefield and if so what changes should be made? Using GenericMOFs forcefield and molecule definition TraPPE.