If the headings are not present in your css and you edit them through the RH css editor, RoboHelp normally adds them to your style sheet. Do you use more than one style sheet and if so, have you checked all style sheets?
When you use a heading style without styling it either inline or in a css, the browser will show the default styling. To make sure this doesn't happen, make sure the styles are in your css. If editing through the css editor won't work, try adding them manually to your css:
You should then be able to tweak them trough the css editor.