Space between header and paragraph.

Which version of RH are you using? Please include the full version number.

Exactly what you need to do is hard to say depending on your project and content. Following are my most likely guesses.

First, generate the output and check if the formatting still looks incorrect when you open it in a Chrome or Firefox. What you see in Robohelp doesn't always match what the generated output looks like (New UI is closer, but still not a guaranteed match). 

If it looks wrong in the output, then the first thing to consider is that a bunch of formatting will have copied across from Word and be applied inline (assuming you did Ctrt+C and Ctrl+V. If you got the content into RH some other way, this may or may not apply, it's hard to know). This inline formatting can override your stylesheet. You may want to remove all this inline formatting, so that your stylesheet applies correctly. Or you may want to keep the inline formatting because that is how your content is styled (I don't recommend this, but I've seen projects set up this way).

If you want to remove the inline formatting, how you do it, and exactly what it does depends on which version of Robohelp you're using. After you remove the inline formatting, if things still don't look right you need to look in your stylesheet at the relevant heading style and the paragraph style. You want to adjust any margin and padding settings you find. 

If your decision is to keep the inline formatting, you'll need to check the inline formatting for each heading and paragraph and adjust the margin and padding individually. (This is why I don't recommend this method, it's a lot of manual labour, whereas updating the stylesheet is quick and easy. 🙂  ) There's also a chance there is something going on with a stylesheet in the project, but what needs to be done is going to be very specific to the project and we'd need more information for this scenario.