Due the complex nature of online advertising, ad campaigns often deliver slightly differently to what it says on the IO. You can either be gracious about this and just let a publisher/network deliver the best it can, or you can demand compensation. If you demand compensation, you may be unnecessarily burning your bridges. While it is pretty common for ad networks to give in and give ridiculous amounts compensation to ad agencies when they let a client down, if you are a stand alone advertiser dealing with a publisher your negotiating power is likely not enough to override the bad feelings you will be creating.
If you are let down by whoever you advertise with, try to let the small stuff go. Those companies which demand compensation for every little thing eventually find their reputation damaged and no good companies want to work with them. It also helps to build up goodwill which means that in the eventuality that you are substantially let down, you can demand and are far more likely to be granted the compensation you deserve.