I'm running 3.0 in OS 10.5 and am having a frustrating time.

I've lined up 4 NN-XT Samplers on the rack with a mixer. Everything comes up on the channel and shows input, but only the 2nd sampler in line will play. Am I missing a key step in selecting between devices? When I go to add additional instruments and effects the sound disappears entirely. Any help would be appreciated thanks.