text box in Power point is disappearing

Hi, I am new to Articulate. i have done one sample course which covers 36 screens. 

i have faced one problem past few days. After publishing or previewed the course, some of the text box/image in the slides are disappearing. Even when i preview a single slide it happens. For text box i am using "peek in" effect. 

first i thought it's some virus problem, and i have done OS re-installation. still the problem is occurring.

i am using Power point 2007.

Please anyone help me out this problem.

Thanks in Advance.