I am using Robohelp 2015
Edited by Moderator to amend version from RoboHelp 15. That will cause confusion in future searches and maybe miss answers that are being sought.
That kills that theory!
It could be that the problem went away with IE11 but I really don't know. With WebHelp, I can only suggest you check your calls against what I have set out in Calling WebHelp.
Could you consider a Responsive HTML5 output. You wouldn't have this problem there. It's not difficult for you to switch but it would mean an initial change to the calls.
See www.grainge.org for RoboHelp and Authoring information