Invalid signature problem

Hi. I use Adobe Reader in my compant for digitlly signing pdf files using certificate feature. Some users randomly get 'Invalid signature' with a red cross. the problem is if any other user tries to sign the document after invalid signature, they can sign it but cannot use the lock function while signing. It is an important requirement for me to get the document locked. The following errror message comes up - 'Unexpected byte range values defining scope of signed date'. Can someone please help as it is urgent.