Adobe Acrobat Reader - problem with signs

Hi.
I have a problem with Adobe Acrobat Reader. I'm using external application to sign a PDF file with certificate sign. There's requirement to sign PDF file by two members(not specified who) of the group. When PDF file is signed two times Adobe Acrobat Reader often shows that one(first) of two signs is not correct. When I open details there's an information 'Identity of sign author is correct' and 'The file was changed or damaged since last sign'. Sometimes there's no notification about a problem with sign and all of them are correct but signing procedure is the same. We tested this for a whole different ways and there is no dependency. I checked PDF file which has a bug in Adobe in different PDF Reader like Foxit Reader or in certificated app to werify sign certificates and there was no notification about incorrect sign.
Please help.