Hmm, in that case thats a big drop. I use notifications mainly, very little banner ads... the notifications are still performing great for me.
Just a thought, if the drop is just recently, then maybe it is because of the after holiday effect where ad revenues are expected to go low.
Secondly, is your app specific to a region or country? If it is, it could be that Leadbolt is out of ad inventory for that location.
Either way, if you decide to look elsewhere, goodest luck!