Adding css file to head

Hi,
I'm trying to add a CSS file to head with Html.AddCssFileParts in my plugin.
When I use this code
@{
    Html.AddCssFileParts(ResourceLocation.Head, "~/Plugins/Widgets. .... /vazirfont.css");
}
and debug, above code run after [GenerateCssFiles] for [Head] and my CSS does not add to the head tag,
and when I use blow code
@{
    Html.AddCssFileParts(ResourceLocation.Footer, "~/Plugins/Widgets. .... /vazirfont.css");
}
It's work correctly and the CSS file added to the footer.

Please explain what is my mistake ?