It can have many reasons.
These days Admob/Google is banning/suspending/limiting like hell. They are literally limiting accounts out of the blue, especially new accounts. It happened to me twice. The worst is: They say it is going to take 30 days, but for one of my accounts I am still waiting and it has been 3-4 months already. Still limited! They are sneaky, because they limited my account just $3 before I reach the payment threshold. It is ridiculous.
Normally it doesn't matter if your app got any API or uses Webview. What matters (normally!) is the traffic. Don't click on your own ads, don't even look at your own ads. But as I said above, it can be any reason....and you got no one to contact, no one you can ask what is wrong. It is a huge problem with Google Admob. They are limiting your account, you don't know whym you don't know how long, you can not contact anyone.
So my solution for this problem was: Remove all ad units on Admob. Sign up for Facebook Audience Network or use a mediation like Appodeal. Monetize your app with them as long as your ad mob account is limited.