Assigning "first page" style - works sometimes, sometimes not

am preparing a book, where page headers must show on current pages, but not on the first page of each chapter. I have prepared adequate page styles, but when I try to assign the “first page” page style by double-clicking on the style name, sometimes it works ok, sometimes it does not. I cannot find any difference in the starting situation.
Does any body have an idea of a possible explanation ?

to be more precise: it works for certain chapters
for others, impossible to change the page style

I have replied with step-by-step instructions with explanations on very similar topic: I have a nine page document. I want a footer on page 2, 3, 4, 5, and 7. How do I do it?

The “First Page” page style is designed for the singular first page of a work. While it can be used for the leading page of chapters I would recommend using a custom “First Page chapter” (or similar) page style instead. This is a good habit to get into because the very first page of a work is often unique in terms of layout, while leading chapter pages usually use a layout that repeats (for each chapter).

Page styles are generally not employed by double clicking on them. The usual method is Insert > Manual Break… > and then selecting the style of the following page. Where content needs to flow from one page style to another the two page styles in question need to be linked by modifying the definition of the leading page style e.g., right-click on the page style entry > Modify… > Organizer tab > set the Next Style to the page style that is required to follow. These two methods allow for greater control over were employed page styles commence and end.

OK. Using "insert>manual break solves the problem.
Thank you oweng
But I would suggest that all methods for assigning a page style work in the same way.

I think you meant to say “all methods for assigning a style work in the same way” as page styles are all assigned in the same manner. It is not possible to assign paragraph styles in the same manner as page styles as they use different delimiters. A paragraph style uses a carriage return to determine where to stop assigning the selected style, while a page style uses a page break or association in the definition to determine the same. The ODF specification would need to change in order to alter this behaviour.

Click in main menu “Format”->“Title Page…” and select proper style of first page at the bottom of the dialog