I have created some additional adverts so that I can do comparisons and see which works best, but since submitting them I have noticed a fall in quality score. One term over the previous 6 months had 1700+ impressions and a quality score of 6. Now I have added an advert but left the original ad in place and left the landing page the same Google is telling me that ad is rarely shown because of low quality score (currently showing as 2). The new adverts are pending review so perhaps this is dragging the overall scores down. Any one got any ideas?
See Adwords FAQ for more details. QS is recalculated after every search. Since your ad under review does not get any impressions, it can't affect the QS. If your new ad had been running, I'd say it's because it's so bad, it's dragging down the QS overall. Note that QS is shown at the keyword level but really calculated at the keyword-ad level. So if one ad is QS=6 and another QS=4, assuming they get shown 50/50, the keyword's QS will show as 5. However, my guess in your case since you say the QS is now 2 from 6, is that having created a new ad, the robot has visited the page and re-evaluated the QS. A low QS like that is usually a sign that your keyword is deemed irrelevant to your landing page so I'd check that. Simply hover your cursor over the speech bubble (under the Status column) next to your keyword.