Steinberg Cubase and MicroTuning

On Cubase 7 / Mac, the Microtuner plug-in does not work properly on ANY Steinberg instrument. It may initially work for a short while but it soon ceases to have any effect. Meanwhile, the other midi inserts do work, so there seems to be a problem with the Microtuner. Unless I’m doing something wrong but I can’t figure out what that could be.