iad/Admob mediation only supposed to work in viewDidLoad method

Supplementing iAd with AdMob for mediation seems to only work for viewDidLoad method regardless of placing the same code in viewDidLoad and another method. I want to implement mediation in my view controller that uses viewDidLoad and GameOver methods. However, only viewDidLoad mediation works. Is iad/admob mediation only supposed to work in viewDidLoad?