UPDATED: May 14 2014
Based on the iPerceptions plan to which you are subscribed, you have the ability to add a number of custom questions to your survey:
iPerceptions Free: 2
iPerceptions PLUS: 5
iPerceptions Premium and Professional: 10
iPerceptions Enterprise: Unlimited
Thus, if you are unable to add new questions to your survey, this may be because you have reached the limit of custom questions that you can program for that particular survey.
However, this limit applies to the number of custom questions that you have included in your current survey, and not to the total number of custom questions that you have programmed since first creating your survey. Thus, if you have already reached your limit of custom questions, and you do need to add a new one, then perhaps you could consider archiving one of your custom questions. This would allow you to create a new custom question.
Click here to learn more about archiving a question