Why the digital sign done with Adobe Reader is considered not valid or non present if I verify the signed pdf with another software for the digital signature?

I'm trying to do a digital signature with adobe reader (pades signature, extension .pdf). When I verify my signed document with Adobe, re opening it, everything is ok, it tells me "valid digitale signature". But when I verify the document with another software for the digital signature (for example: aruba sign, ****) it tells me that the signature IS NOT VALID, or NOT PRESENT. Which is the problem of the adobe signature?