Matilda - That's not really PDMA. PDMA is about a specific action by a specific mod. I think your question is general enough.
Allow me to try and answer it.
We do have a set of rule, however we all interpret the rules differently. We also have different opinions and viewpoints where we see issues seperately than the other mods. In OT, it's especially evident. That's just the nature of OT. Opinion's vary. So the mod you see online might very well agree with the post you reported, and the mod that ends up dealing with it may agree with your point of view vis a vis the post in question.
I would say that if this is a distictive pattern, I would PM the moderator that you think is not handling it and ask them why. There could be plenty of reasons. Reasons that have nothing to do with whether or not you agree with the post. First and foremost, they could be offline. Sure, it says they're online, but keep in mind that it takes fifteen minutes for me to walk away from the forums (close out the windows with CFC open) before it shows them as offline. So sure, they may appear online, but they may not be. Also, we get Emails, not PMs regarding reported posts. It could very well be that the mod doesn't have the email website/program open at the time the reported post is sent out.
Now, of course, there's gonna be times where they are online, and stay online, and they have their email open. In that case, it could be that they just don't see a reason to moderate a post. Like I said, opinions vary.