First off I'm a Microsoft Gold Partner, .net community evanglist, and MCP. I've informed adCenter and created partner trouble tickets for well over a year about my issues of my ad not showing up for my keywords.
What they keep telling me lately is, because the keywords are considered "low performing" it deactivates those keywords. First off, my Google ad never has this problem. I've been getting the runaround on what the problem is and what the fix is. I was originally told my low budget created the problem. However, my ad has never come close to reaching my budget, much less accumulating over $2 in clicks (I've set the budget up to $150/mo).
Yes my keywords don't get searched much. But it's the number 1 search term for my product! I might be in a niche market, but that doesn't mean I should get substandard service, especially when Microsoft knows they are under the gun for trying to keep pace with Google.
I'd be curious to know if anyone else has had issues and what they are. Only then can we help encourage Microsoft to fix the problems and give some true competition to Google.