Don't know if this is the case here, but some programs restrict the usage of their plugins to their own program. For ex., Sonar's DSP-FX plugins - although rendered in DirectX format - can only be used in Sonar. When I try and use them in Wavelab, I can see the plugins in the DirectX plugin section, but I can't actually use them.
Conversely there are some VST plugins supplied with Wavelab that I can use in Sonar, while there are others that I can not.
Perhaps that is the case with Cubasis as well. I wouldn't at all be surprised.
Obviously any plugins that are purchased as a plugin (rather than supplied with another product), should work in any program that can handle the format.