This happened to me a while back.

The cause of my issue was that I had inadvertently inserted the "Pipeline" plugin on a channel. Pipeline forces realtime rendering because it's for external effects which do their thing while the song plays/renders. There have been a few posts of late about this and there may be other causes, so if this isn't your issue try searching for "rendering", render, realtime, etc.

Lastly, create a signature* as it will help us help you zero in on your problem.