5G NR PDSCH Throughput MIMO 4x4 and 8x8 with 4 layers not providing a throughput and High BER in 5G Toolbox

Hello. I have question need to be asked. I am simulating MIMO-OFDM for 5G NR for MIMO 4x4 and 8x8 with 4 layers, so it is just SU-MIMO (No beamforming and No Massive MIMO) using 5G toolbox. When I simulate to compute BER and PDSCH Throughput, It gives me a high BER from 0 dB to 25 dB SNR. When I tried 4x4 and 2 layers, it gives a very good result in BER and Throughput. So, why the 4x4 and 8x8 with 4 layers gives a very high BER and throughput is not computed? Any information please? I have attached the m-files here.