There is so much going on within the terrorist groups around the globe that no one individual or even national interest can be aware of it all. So if these terrorists want to gather and not let any large western nation know what is going on, they are going to be able to achieve that goal.
It is possible for the US to not know about such meetings simply because we are on the outside always trying to view inside. When the US can infiltrate such meetings, they will. The public has even less knowledge of the meetings that the US government would, so there could be action under the radar that we should not be privy to.
Regardless of what many may think the US is very involved in knowing what is going on in foreign affairs. Now whether or not they make that public information is a whole different story. Intelligence is not something you go around announcing to the whole world. To say America doesn't know the doing of terror groups is absurd.
Given that the media reported on the large alQaeda meeting, essentially as it was going on, I find it hard to believe that the United States, on some level, is not aware of it. After all, the media seems to know about, right? Just because you don't see the United States, doesn't mean they aren't these. Some things are covert for a reason.