I gave up on text ads people don't click any of these ads at all. We use image ads from google. We have been getting paid per 1000 views.
I don't mean to be judgmental here, but you do see that's practically fraud don't you?
Forums hardly convert in the first place unless you put very targeted material that is relevant to the forum, and visitors/guests are more likely to click than returning members, which probably make up the bulk of your traffic.
Since you weren't earning with text ads, it meant people weren't clicking, therefore there's no interest in the ads. Replacing those with image ads merely to get the money for the impressions, especially when you know there's no clicking, is pretty much defrauding the CPM advertisers out of their money.
If Google measures the ROI for those image ads and sees the advertiser is getting no value for those impressions it's possible you'll stop seeing ads someday soon and/or get booted from AdSense for irrelevant traffic.
Not saying it'll happen, but it's a possibility, so if you value your AdSense account on other sites you may operate, I would consider finding new ways to monetize that forum ASAP.