Sounds odd to me, but could be a graphical error of some sort, do you get any error messages? Make sure the game is updated to the newest patch also do the same for your graphic card drivers.Do some searching on Google to see if others have had the same issues.
Did some searching for you.
Portal 2 Bugs & Issues Thread - Steam Users' Forums
List of game breaking bugs, some include some elevator scenarios hope that helps.
Also in future it would most likely be best if you made an account on the steam forums for any issues with valve games, there is often a lot more dedication and users of course that have played the game to help you out.