Two things make the biggest difference:
1. Allow tracking permissions
When you launch a new app or game for the first time, you'll see a pop-up asking permission to track your activity. Tap Allow.
Without tracking permission, the offer provider can't record your progress and you won't receive your reward.
2. Only do offers for apps you haven't installed before
If you've installed an app previously even outside CashCow your tracking data is already registered. Reinstalling it won't count as a new user, and your progress won't be tracked.
To stay safe, only pick offers for apps and games you've never downloaded before.