I love the top attack, but I've seen this several times now, both in our MP gameplay, and in SP mission testing in the editor.
TAB simply does nothing. Ctr-TAB switches between direct and top. Pulling the trigger in either mode does nothing. Yes, it's loaded.
Tonight in a mission they worked for a few shots, then stopped. Not every player has the issue at the same time. For instance, one guy had the issue at one point tonight, I took his firing position and confirmed that I could seek and lock on the target, we decided I would take the shot, but when I sighted in again from the exact same place, TAB didn't work any more. Poof, just like that.
Is there a known-fix for this? If not, Vanilla is just fine.... out goes the javelin pbo.