One of the options within ‘Survey Options’ allows you to alert participants if they try to proceed when they have left questions unanswered.
This highlights questions on the page they have not attempted to complete. (see screen shot)
The 'blank answer' alert option
Please note that due to ethical reasons you CANNOT force participants to answer all questions before progressing to the next page- all iSurvey can do is alert them to the fact that they have missed some, and if they wish they can attempt to complete them.