There can be multiple reasons why you can't start a video call (the button doesn't turn blue). A poor Internet connection can be one of the reasons or the fact that one of the devices that are engaged in the conversation doesn't support video call. If you are sure that none of the above reasons applies to your case, update the application or clear its cache. This can be done by going to menu > Settings > Applications > Manage Applications > All tab, tap on Snapchat and select Clear Data and Clear Cache.
