when you make a change to your page, you have to wait a few days to see if it helps you in the SERPS.... when you make a change to the keywords on your page to display more appropriate ads..... how long do you have to wait until adsense notices? Does it do it instantly each time the page is loaded? or is there a wait time?
When the adsense bot crawls your site, generally you can do this by refreshing your page. However more relevant ads might show up after a day or two
Watch your logs - when you see the mediapartners bot stop by the page, it will be retargeted. However, how frequently the bot visits depends on a number of factors, just refreshing the page won't neccessarily work.
For me it takes anywhere from an hour to 24 hours. I manipulate my keywords even though my keywords are still relevant to my site. If they weren't I would receive a warning but I haven't. My tip is to place the keywords near the codes because that's where the bots scan first. It really does work.
I was making some changes to a directory yesterday and could see the ads subtley changing as I worked. May have been a coincidence (after all the ads can have max impressions can't they?) but the changes seem to have stuck.