Hi-
At the moment, Instant Phaser Mk II is VST2 only. VST2 is known to have issues with sidechain in Cubase, as the method for sidechaining in the VST2-spec is a bit of a hack. I suspect that the other plugins you are using with sidechain are VST3 – please let me know if that is not the case.
You might also try this video, as it outlines how to set up VST2 sidechain in Cubase, though I'll admit it is quite involved: https://www.youtube.com/watch?v=KI1s6HnTccM
We are working on a VST3 version which will simplify/fix the issue – however I cannot as of now promise a release date.
-Tom
