fold_page_list on blogpage

I have several pages and subpages. One of the pages is a blogpages (the php code for a blog is on it., the loop) I have the parent items in the menu, when I select the parent, the submenu shows, with all items. WHen i select one of the items the submenu stays, except for one, there the submenu disappears and only the mainmenu (the mainpages) stay.
The only exception is that that page has the code with the loop for blogmessages. Is there a way to use a blogpage and also show the submenu-item? I hope it is clear what i mean. I’m clearly not native english 😉