Cannot talk to ATC

Help needed please. Have returned to FSX after a long illness. Reloaded all the software and all is working fine….except…when I tune into an ATC frequency I am told by another(helpful) pilot, that I am transmitting on UNICOM …but I am not.
My radio stack shows my required frequency in the active section but UNICOM is not in the standby. Everything seems to be topsy turvey
Can some one help please?
Thank you
Donald