I think I figured out why it just showed "Your ad is probably being shown at times, but was not shown for this particular diagnosis." and never show it.
It's a fairly popular affiliate program and there are some very big guys competing for the keywords.
Every time I boost the max CPC, it showed my keyword for a few hours and then 0 impressions again.
Created new campaign with higher max CPC, showed again a few hours and then 0 impressions again.
It got to a point where I paid 40 cents on avg for a click, but max CPC was set to $100. Right now, even if I put max CPC at $100, it doesn't show my ad.
The other guys are just outcompeting me with huge bids, even if it means paying for the occasional max CPC clicks from google.
The Keyword search volume for May is 8000 in total(4k desktop, 4k mobiles) for a few countries combined, but it is growing very quickly.
@IFoundYou2000 do you know if there is any way to compete in that scenario with the big boys? Right now I created my own landing page and put ads up for that, to see how much higher CPC cost is and if I will get enough clicks for any conversions.