Client signs docs sent but it then takes hours to return to me

The delay has been getting worse and worse over the last few months. I send a doc for signature, the client gets it, signs and then it can take anywhere from 30 min up to hours for it to return to me. For example, I sent 4 docs to a client today. She notified me that they were all signed. One came back within an hour, and three hours later I am still waiting for the others. Is there a setting somewhere that I need to change?