Do you re-estimate story points when they are finished?

I have a question for you. In one of my teams sometimes happens that a user story is not finished and we need to move it to the next sprint.
Does it needs to be re-estimated?, so lets say that only testing is missing, and the original estimate was 21, and now it is 3.
So what happens with those 18 points, should i count them as finished work for the previous sprint, even if it does not meet the def of done(testing is missing) ?
or should I forget about that. If I forget about them, there might be less points in the sprint and you know, Management will ask hey what is going on. This problem gets worse if
instead of 1 user story I have 3. What is your approach, for now i am trying to reduce the amount on big user stories, because this is happening specially when we have
user stories with 21 story points.