Genesis/Dynamic and Nav Placement

I have been asked to create a site using Genesis (fine) and Dynamic (help!)
I need to place the main navigation in the middle of the page.

I used the following code:

.menu-primary li {
float: none;
display: inline-block;
} 

.menu-primary ul {
text-align: center;

}

This worked fine, then suddenly stopped working!
When I resize the window, the menu moves to central position for tablet and mobile, but insists on sitting on the left full screen. I haven’t changed anything in the @media queries.

Can anyone help?