Ads not appearing

Anyone know a reason not listed on the adwords help site?

[ShootinBlanks]

A company I work with has a number of keywords on adwords, but when we search for them, our ads don't appear on the SERPs, sometimes there are NO ads at all, sometimes one or two of our competitors. I've gone down the list of possible reasons, and none seem to apply. Has anyone else seen this lately? Any straws to grasp at would be appreciated. I'm stumped.

[netmeg]

Go into your AdWords account, into the Tools section and click on the Ad Diagnostics. Usually it's a case of the budget not being enough, but possibly you will find some other reasons.