How do I improve ad fill rate?

Yesterday my ad fill rate was 14%. Does that mean only 14 percent of the time my game served ads?

I use mopub and admob. Do I need to create accounts with other advertising companies other then admob?

Any other ideas how to increase that?

Just wait, I had an admob fillrate of 2% the first day. Now it already has gone up to 41%.

You could also use another kind of advertisment. For example, Tappx uses a kind of "ad networks waterfall" and that way you lose less traffic. They work with several ad networks and your "traffic" moves from one net to another until there aren't more nets (sorry for the lame explanation, it's a brief resume of what I understood when they told me about it) But yeah, it works great! They also work with cross promotion, they gave me some advice for my marketing campaigns,so I am pretty happy with them. Just a suggestion