latency

I'm getting latency on inputs on a song. I'm half way through, recording was fine before & now I get a slight delay. If I keep this song open & open another blank song & create a track to record onto, there's no latency. When i go back to my track the little coloured spinning wheel spins for a few seconds & I'm back to latency world again - so the Preferences - Audio are the same for both songs, but the one with lots of stuff recorded now has latency. I had put a fade by automation onto the main stereo fader/track for a ruff mix, but I have removed the automation - could this have caused it - if so, it should have gone when I disabled it

Any clues please?