XLPagerTabStrip: Dynamically adding subviews to tab's view controllers wouldn't display except under first tab.
Please download the project and run it to see the problem:
https://github.com/farhan101/TheProblemView
First Tab: Under View One tab you can see a spinner added dynamically
Second Tab: Under View Two tab same spinner is added but it wouldn't show

PS: As you can see in the source code, both view controllers are instances of the same view controller class.
You need to use the parent view's bounds instead of frame while creating ViewToAdd.
let vu = ViewToAdd(frame: self.view.bounds, yFactor: 2.0)