This is likely because you did not register the renderer in your platform target app's MainActivity.cs, AppDelegate.cs or MainPage.xaml.cs.
If you're using a version older than R2 2017, you must manually registering the renderer. Please follow each step on this page
in the documentation, at the bottom is the step where we state you register the renderer
If you update to R2 2017 and later, you no longer need to manually register the renderer. Additionally, there have been significant improvements in the R2 2017 release that make the update worthwhile.
Let us know if you have any further trouble or open a support ticket here
Lance | Tech Support Engineer, Sr.
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