Issues with Html in posts

Hello all,
We are having some issues with a new blog we are trying to get up and running,

when we create a blog post in wordpress with all text formatted as we would like – when we generate a preview non of the formatting is pulled into the post.

I had a custom theme built for the website.

So even if we write a post in html and paste that into the ‘text’ screen for a new post when we press save all of the the html, like “<br>” is removed.

Can anybody help or explain why?

Thanks