Based on the information you have provided, it is hard to advise anything specific regarding your problem. I guess you first have to check the geometry of the structure, do proper volume and ion relaxation, using the appropriate DFT functional and basis set with possible corrections that might be needed for your system; check the convergence with respect to computational settings.
You may also need to check the reference you are using to validate the computational results; look for different literature sources and compare if they are consistent.