Ran in to this issue while trying to implement the new DFP tag. I noticed this when for a certain slot, the ads were not rendering. One can see this in the google ad console which can be forced to open by appending google_force_console=1 to the url.
This is causing the ad to not render even though all the slots and display code is correctly added.
Found the answer here
Solution is Look out for if you are passing any integer as a value since GPT only supports string.
Additionally the article also mentions a nice utility called Charles for Mac which looks like a good alternative to jsfiddle.