Don't assume that lack of clicks is due to great / addictive gameplay. It also may be that your audience or demographic is not prone to clicking on ads; the ads contextually are not well matched to your audience; etc ...
With your particular challenge, as you hit higher audience levels, maybe think about sponsorships / branding opportunity where you can sell the 'eyeballs'. Since you are currently making nothing from the ads, maybe dump them and play around and give some sponsorships away as a trial to brands that you think are cool and well matched to your audience.
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Second the comments on paying for extended play, but maybe instead of levels (which you say don't exist), pay is for extended time per day. (free 60 minutes cumulative a day, extended no daily limit)
With your particular challenge, as you hit higher audience levels, maybe think about sponsorships / branding opportunity where you can sell the 'eyeballs'. Since you are currently making nothing from the ads, maybe dump them and play around and give some sponsorships away as a trial to brands that you think are cool and well matched to your audience.
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Second the comments on paying for extended play, but maybe instead of levels (which you say don't exist), pay is for extended time per day. (free 60 minutes cumulative a day, extended no daily limit)