You should look at the solver and CVX output and optimal variable values after each iteration, and see what the resulting inputs are for the next iteration. Perhaps the optimal values, and therefore input values for the next iteration, are getting wilder and wilder with each successive iteration, until eventually, failure occurs for some reason.
You should look at the stated reason for failure. It might be that he problem is assessed infeasible, or unbounded, or the solver had numerical difficulties.
It may be that (with your starting values), you are getting 'lucky" for certain values of SNR, and not getting lucky for high values.