function bookmrk.GetByTitle stoped working as it should

function bookmrk.GetByTitle stoped working as it should. It worked properly early. We tried to reinstall then Acrobat, but it didn't help.

Here is VBScript

Set MainPartDocument = CreateObject("AcroExch.PDDoc")

MainPartDocument.Open("C:\1\1.pdf")

Set bookmrk = CreateObject("AcroExch.PDBookmark")

'on the next line function returns False, although the "bookmark" exists. Why?
res = bookmrk.GetByTitle(MainPartDocument, "bookmark")

msgbox res
if res then
msgbox "bookmrk exists " & bookmrk.GetTitle
end if