There are a lot of reasons behind, I will try to list all of them as it's difficult to know exactly without a look into the account.
- The bid - When you set your bid, you're telling Google Ads the maximum amount you're willing to pay for a click on your ad. How much you actually end up paying is often less. But if it's too low it will not trigger.
- The quality of your ads and landing page - Google Ads also checks how relevant and useful your ad and the website it links to are to the customer. Our assessment of the quality of your ad is summarized in your Quality Score, have a look there as well
- The Ad Rank thresholds - To help ensure high quality ads, we set minimum thresholds that an ad must achieve to show. From what you say you shift to max clic, so technically your are not limited on the bid part?
- The competitiveness of an auction - If two ads competing for the same position have similar ad ranks, each will have a similar opportunity to win that position. As the gap in ad rank between two advertisers’ ads grows, the higher-ranking ad will be more likely to win but also may pay a higher cost per click for the benefit of the increased certainty of winning.
- The context of the person’s search - With the ad auction, context matters. When calculating Ad Rank, Google check the search terms the person has entered, the person’s location at the time of the search, the type of device they’re using (For example, mobile or desktop), the time of the search, the nature of the search terms, etc.
- The expected impact from your ad assets and other ad formats - When you create your ad, you have the option to add additional information to your ad, such as a phone number, or more links to specific pages on your site. That's the ad assets. Google Ads estimates how assets and other ad formats you use will impact your ad's performance. It's becoming more and more important. I would check first there.
If you can't find it, you can always contact google support directly from the account to clarify.
