lower amount of offers during installation = lower rate per install.
Look how many offers amonetize bundles for 1 US installs. I counted 13. How much do you think they charge advertisers for 1 offer and how much do they pay publisher for 1 US install?
Contact advertisers. Make your own installer and bundle offers.
Which pop under you are using ?I use also pop-under and these things, it's great revenue. Contact me via private message and I will send you site where you can register.