Reuse wire nut when adding electrical outlet?
Wire nuts have improved in leaps and bounds in the past 20 years, so if a wire nut is clunky or finicky, I toss it out immediately and replace with new.  I will only reuse one if it seems to be performing "tip top".  

It isn't like you have to evaluate each nut individually.  There are probably only 1 or 2 types in use in your building, you soon learn which aren't very good.

Also, wire nuts are only expensive when buying them in small lots, and you're paying for packaging and the big-box store premium.  Once you are buying them in bulk, they're too cheap to care about.  

There's also a question of size.  Wire nuts have a working range, and it's best to be sort-of in the middle of their range.  For instance I like yellow wire nuts for 2 wires, maybe 3; for 3 or more I  prefer red.  On the fringe of their working range, they will often give unreliable results. 

Lastly, don't use wire nuts for aluminum, even if they're purple.  Use Alumiconns and an AFCI breaker.