@cinder , Sally, The 1st link below shows how the ball covers are held in place by the plastic type threaded trim nuts. If you are missing the nut or trim cover damaged, threads stripped,etc., you may have to order a replacement part. 2nd,3rd link may also aid in trouble shooting. Link #4 is where part may be ordered from if need be. Make sure to check your warranty to see if you still have coverage on your Dyson. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
https://www.youtube.com/watch?v=Bl7dPL3TWZ8
https://www.youtube.com/watch?v=1XTn5q0OLmk
http://manchestervacs.co.uk/DysonForum/index.php?topic=1030.0
https://www.dyson.com/support/dc14/dc14-animalus