We found that insufficient sampling of the dihedral degrees of freedom in the amino acid side chains can lead to poor convergence in the deprotonation free energy profiles, as was also observed by Klimovich and Mobley in simulations without constant pH. We traced the lack of the dihedral sampling to the barriers that separate the minima in the torsion potentials. These barriers are too high to reach a converged sampling of the dihedral free energy landscape on the timescales of typical constant pH MD simulations. Because the interaction between the titratable group and the environment depends critically on the dihedral angles of the side chain, a lack of convergence in these dihedral angles also affects the sampling of the protonation states.