This is not a NAML issue as Nabble's own code only affects the internal working of a forum.
What happens when you insert the Nabble code into a web page is determined by the page that receives the code, NOT Nabble.
Often, it turns out that those with problems inserting their forum are using visual "drag and drop" techniques to create the space into which to insert their forum. They do not realise that by using such techniques they are setting limits to the space allowed, typically not allowing it to expand vertically or setting a space too narrow for the full width of the forum to display in full.
You don't say what web design software you are using for your main site, or where it is hosted, but almost certainly you are running into that problem. If this is the case then you need to contact the support service for the web design software/service you are using, as the problem is with the techniques being used to create the space for the forum and not the forum itself.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me!