I tried making a CQ on 20 m, 14.060 MHz (the QRP CQ frequency) and parallel to that I had a WebSDR on my computer just to keep an eye on the band. I saw someone responding on the WebSDR, which is rather close to my hometown; however, I couldn't hear him on my receiver and the signal on the WebSDR was too vague to copy. 

What should I do in such a situation (apart from improving my receiver)? Should I just send my CQ sequence again and hope the other side understands that I can't hear him, and stops trying or increases power? Or should I send another sequence to say that I know there is someone, but I can't copy him?