We are going through the exact thing here with NVR.
Yesterday we were revising and she had the paper and calling out the answers and got 2 wrong and yet she had got these wrong in the practice. Like everyone said more practice, with my DD she is now sprinting and finishes with about 20mins to go. She just wants to watch TV so I have told her if she keeps getting those mistakes she will stay in her room until the time is up.
Evil I know but it annoys me when she gets things wrong that she could have easily got right.
Have you picked a trend of the ones she is getting wrong? You may need to focus on those. I try not to do too many papers but use them so that I know the areas we need to focus more on and when I think we have progressed give her another paper.