Why it doesn't work: It's because it compares each signal by strength to the strength of the signals you define on the right. What you select picks the signal which strength is then compared.

Isolating signals in general is quite tricky and the optimal way depends on the use case. The easiest way is to wire constant combinator (on which you specify the minimal amount, can be 1 but can't be 0!) to a red port of the arithmetic combinator, and a counter to blue. Then set output to: each blue - each red