LyX ⇒ Problem with alternating Headers

[rc3002]

I'm using fancyhdr package. I added in the preamble the following commands:

Code: Select all

\fancyhead[LE]{\sffamily\nouppercase{\leftmark}}
\fancyhead[RO]{\sffamily\nouppercase{\leftmark}}

That is, the section name should appear on the left side header on even pages (LE), and on the right side header on odd pages (RO). It works quite well, except for some pages which keep the place of the previous header (i.e., there are some odd pages with header on their right side, as in the even page before). Does anyone have any idea why this occurs? I was thinking that it has something to do with me using \vspace commands. Maybe pages with \vspace{-kpt} (where k is some positive number) cause the following page to be considered as the previous one?

I'm using LyX 2.0.0, and I'm working on a document class (in Hebrew).