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