You should look at the solver and CVX output. Does CVX say the problem is infeasible? Or perhaps the solver failed? If you are not using CVX 2.2 with Mosek 9.x or 10.x, and are not using CVXQUAD with exponential.m replacement, then CVX"s unreliable SUCcessive Approximation method is used, which may fail, even when an optimal solution actually exists.
You should use CVX 2.2 with Mosek 9.x or 10.x, else follow the instructions at CVXQUAD: How to use CVXQUAD's Pade Approximant instead of CVX's unreliable Successive Approximation for GP mode, log, exp, entr, rel_entr, kl_div, log_det, det_rootn, exponential cone. CVXQUAD's Quantum (Matrix) Entropy & Matrix Log related functions to try to prevent solver failure.
Also, make sure the numerical scaling is good.