Several people fail to put the keywords in the ad text (the heading and the body text). This makes their ad highly subdued (because if you would have had put the keywords in there, these would have been highlighted). Also the searcher may feel that your ad is not relevant enough.

You must try to put the keywords and an active verb in the heading for maximum impact. Example - use Get Fresh Adwords Tips when your keywords are ‘adwords tips’. This gets the words ‘adwords tips’ highlighted and also indicates to the prospect that you offer what he/she is looking for, thereby improving the chance of a clickthrough.