In each forum you will find some Sticky Topics to help you get the best response. Before you Post is one of them. Please take a look at it.
Your post does not tell us which version of RoboHelp you are using or what output you are creating. If you will add that information, then I am sure someone will be able to help you.
It might also be worth adding screenshots so that we can see the problem to which you refer. When you reply, there will be a camera icon that allows images to be added.
See www.grainge.org for RoboHelp and Authoring tips
I'am using RoboHelp 8.
For the program where the search field is OK, I create WebHelp (primary layout).
For the program wher the search field is too narrom, I create Microsoft HTML Help