Hello Oriol,
I am another staff checking in and I have taken a closer look at your issue. There are things that I have noticed on your site which I think are the reasons why you are having this issue.
Issue 1:
Your actual header height is 76
pixels and yet in your theme options settings, Pro > Theme Options > Headers > Top height, you only have set it to 70
pixels. Please have it increased because the JS scroll events will compute the top height.
Issue 2:
You have added the element IDs in your row elements which is not the basis of the One Page Navigation. To properly use the one-page navigation, the IDs should be added to the section element. Therefore, your page elements need to be structured into sections like this:
Issue 3:
To properly implement the One-Page Navigation, you will need to create two set of menus. One menu will be used as your primary menu and the other menu will be used exclusively as your One Page Navigation menu. In your primary menu, your custom link will have the absolute URLs while the One Page Navigation exclusive menu will only have the relative URLs.
Primary Menu absolute URL http://example.com/#anchor-name
:
One Page Navigation menu with relative URL http://example.com/#anchor-name
:
Hope this helps.