Aux / Send channel behaviour Question....

Using Logic X, I have a project which uses multiple sends for AUX channels for FX plugins.

Sometimes, when I SOLO only the AUX CHANNEL, I can hear the wet FX of whatever is being sent to it.

And sometimes when I SOLO only a different AUX CHANNEL, I cant hear the WET fx unless I also solo the track that is being sent to it.

Why does this sometimes work and sometimes not?

I cant find the reason or a reliable method to make it behave the same every time

thanks