I don’t think most people set out to be familiar with click tracking line items for DFP but sometimes in the line of duty, you become an expert.
The use case for myself was to have third-party tracking in emails that was free. A quick internet search revealed this article: https://support.google.com/dfp_premium/answer/1212244?hl=en. Surprisingly it is a very thorough description from Google and easy to follow. After about 10 minutes, I had DFP click line item tracking all setup and I was at the point below:
Naturally, I was excited and quickly pasted the URL into browser. The URL loaded and then…white screen of doom. First, I thought it just took some time for the creative to resolve and populate (which it does). Reexamining further with my co-worker, we noticed that the URL is non-secure, so I updated and presto it resolved.
So long-story short, if your DFP click tracking line item is not resolving check to see if the URL you are calling is secure and it should hopefully solve the issue.