If you have multiple campaigns running in a single game, you can use the Campaign Logic section to adjust each campaign's priority:
This setting controls the order in which impressions will be served when more than one campaign is running in a given app.
If you find that Game X never seems to deliver impressions from lower-priority campaigns, start by checking the list of campaigns it's publishing; you can get to this list by switching to the game with the app switcher, expanding Campaigns in the left-side nav bar, then clicking Publishing:
Once you're there and can see all the campaigns running in the game, try to imagine how an actual player would interact with them. If your game only occasionally calls for an interstitial, it's very possible that users would never have the chance to advance past the higher-priority campaigns to see your lower-priority ads.
To drive more traffic to your lower-priority campaigns, you should either bump them up in priority or decrease the number of impressions served by higher-priority ones.
To accomplish the latter, consider turning off higher-priority campaigns at some locations to allow lower-priority campaigns to pick up the slack:
In the screenshot above, the campaign will not serve impressions at the Default location.
For more in-depth information about priorities, see our FAQ here.