Well... after reading a little bit the manual and having a little bit of imagination, I have the following theory...
There are two types of campaigns in Adwords, either paying per click or per time the ad appears.
Using pay per click, Google needs some data (history) about each of your ads to figure out which would be the hypothetical cost per impression of your pay per click ad (i.e., once the ad has been published 1.000 times, and knowing how many times you actually paid for it -there was a click- it can calculate the average cost per 1.000 impressions).
Google knows that if it prints a pay-per-impression ad in a web, they will get money for sure. If it prints a pay-per-click ad... who knows?
So I have the theory that every time you have a new ad (or edit it), it needs quite a long time before it can directly compete with pay-per-impression ads. So you only have to wait.
My experience says: if you want to modify anything, the best way is not touching it but creating the modified thing WITHOUT deleting the old one. Leave it there for some days and when it starts to perform better than the old one, it's time to delete it.
Sorry if I'm stating something very obvious, but I felt very smart when I convinced myself that this was the answer to this problem (:-)=
BR Iván Arias Rodríguez