Adding of annotations

Hello,
I have a little issue with adding of annotations to a page. The issue is, that a code, which I use, is working, however in case, that a PDF-document view is increased in its size, the code stops to work. When I decrease the size of the view, everything works fine again.
For adding the annotation I use this code:

page.AddAnnotText(new float[] { x, y });
page = vPage.GetPage();
int annotCount = page.GetAnnotCount();
Annotation textAnnot = page.GetAnnot(annotCount - 1);
textAnnot.SetPopupText(noteText);
textAnnot.SetFillColor(Page.highlightColor);
textAnnot.SetPopupSubject(String.valueOf(timestamp));

page is an object of class page acquired by a method vGetPage (currentNbrofPage).

Do you know where could be the problem? Many thanks.

increase View size cause vLayout, which re-layout all pages in async way.
in this case, page object did not avaible.

decrease View size mostly not cause vLayout.

Thanks for your reply. Is there any way how to add annotation when the View is increased?

wait untill page rendered.
in PDFReader.OnPDFPageDisplayed()
check vPage.IsFinished();