On a couple of flights, I have used the ATC window to try and request a different runway/approach for landing, after I have received inbound clearance and landing runway. Each time, I have been met with total silence from ATC.

Has anyone else experienced this? Am I doing something wrong? Is there a work around?

Thanks.

Yes there is (a workaround). Step 1) Put the emergency frequency (121.5) or anything else on comm, 2) pretend you are talking to atc, 3) do whatever you need/want, 4) be moderately satisfied.