USB 6008 using all analogue input channels simultaneously

Hi all,

I'm using NI USB 6008 to record some data, from its analog input channels. I need to use all 8 input channels at once (A0-A7). When using A0-A3, everything works well, but when using A4-A7, their signals also cause a signal change in the A0-A3 channels (but not vice versa).

All channels have grounds connected. I know A4-A7 are meant also to be used for differential measurements, but according to NI manuals, it should also be possible to use them all as individual input channels. So any thoughts how to use them all as independent input channels, without interference?