1) Peek at the java generation for your activity, both in the case of using real property-set, and using a java step instead, and do a text-diff of the two. See if there any calls showing up in one and not the other for handling triggering. If both look the same in that regard, I would expect the triggering to be the same for both.
2) If the security tab of the activity shows the activity to be an "onchange" type, then triggering won't occur. (What other types don't do triggering ?)
If the above still doesn't help you resolve your problem, then we'd need to know more about what kind of triggering you are expecting that is failing to occur.