Is it an instrument or an effect? My experience is limited to effect units. For some reason Garageband only starts calling the render block when you start playing your track, and when it has actual audio data to send to the plugin. So playing an empty track won't result in it starting the render block. Freezing parameter sliders: I have seen those at some time. As in : Garageband knows all the parameters, but they are all set to 0 and you cannot changes them. I am not sure what causes it, but you might want to try again in a blank Garageband project. Garageband does state saving on its projects ( by reading and setting the fullState property ) and I have a feeling that this sometimes messes up things. Especially on a plugin that is still in development where old fullState values might no longer be compatible with your new version.

Many thanks for the confirmation Nikolozi.