Why Facebook Ads Underdeliver after restarting them?
According to Facebook rep, Pausing an ad set and restarting it again tends to ruin Facebook's auto optimization algorithm. They didn't explain way, but apparently that's the way their monkeys programmed it.
What To Do Instead?
A little workaround would be either using the Power Editor, or Ads Manager you can duplicate your winning ad set and start a fresh new duplicate. instead of re-starting the old one, that will work quite well.