If you switch to 'Design Mode' while you are creating the flipcharts the hidden objects will show on the flipchart page with a red border around them. The red border is showing the object has an action attached to it. which in this case will be "Hidden".
Then switch back to 'Presentation Mode' to present the flipchart.
Switch to Design Mode by clicking on the snowflake (or gear) in the top right ActivInspire so it turns red. Click on it again to return to Presentation Mode, which is when the snowflake/gear is blue.
Hope this helps.