spectral subtraction for stationary noise
In almost all practical applications of spectral subtraction it's unrealistic to expect that the noise can be completely eliminated. Even if this were possible, the price to be paid would be severe distortion of the desired signal.

What you can try is tweak the parameters. The most obvious parameter to tune is the (over)-subtraction factor, which determines how much of the estimated noise is subtracted from the noisy spectrum. It is not uncommon to have over-subtraction factors of around $2.0$ or $2.5$. Of course you have to make sure that the resulting estimated speech (magnitude or power) spectrum remains non-negative.