Your problem is most likely down to the way Poser names props. If you load a box prop it will be named "box_1", if you then load a second box it will have a diffrent name "box_2". This should not stop morphs themselves from working, but it will stop ERC from working in any but the first instance of the prop, this is because the name the slaving code is pointing to is only correct for the first instance of the prop. This also has consequences when loading morphs into props from pose files as delta injections.

It's not really posible to say more without knowing exactly what you are doing, or even better having an example of one of your props to look at.