It may be my broker. I'm placing too many orders that don't show up in Level 2. It's intermittent though. I think optionsXpress uses different MMs for every order. And, some of them don't post the order whereas others might. In those cases where my order is posted, it seems I get filled immediately.
I believe you are on the right track with MMs not filling orders. They are going to be locked with tons of UNGS shares and tons of UNGS divvy of every flavor. Waiting on someone with the handle Goldilocks to join our message board. I'm thinking they have lots of tasting to do from a bottomless bowl.