FFT based adaptive MVDR beamforming

I have a small question on FFT based adaptive beamforming based on Spectral Matrix Inversion technique. I have estimated the Spectral Matrix (Sxx) from my data by dividing it into blocks and averaging them for each bin, to get sufficient rank on the matrix. To produce the MVDR weighting vector (8x1 vector in my case since I have 8 microphones) for each bin, I multiply the inverse of Sxx for each b