Staff split but synth not - how to fix?

Beginner so maybe I am missing the obvious. I opened a SATB template and split each staff to create SSAATTBB but score and mixer show only one S, A, T, B per pair of staves. How can I get a unique voice/channel for each half of a pair? What did I do wrong at the outset?

EDIT: Should have used the Instrument tool to add parts at the beginning but created a new SSAATTBB doing so and was able to copy the 4 split staves into the 8 separate staves en masse. Now I have one mixer channel per stave - great!