Changing Pitch of pre recorded music track

I am all of a sudden having trouble with changing pitch on pre recorded music tracks. I did several tracks last week and it worked fine. I changed some up a little and some down a little. However, when I tried to do more today, I cannot get it to work. Just as before, I locate and import the track, highlight it, then click on the effects word and select change pitch from the drop down box. I select the start key that the song is in and the key I want to change it to. I check the box for slow high quality change and then click OK. Nothing happens after that. Before when I did this process, it went right into the slow change and out came the song changed perfectly. Am I missing something? It worked fine last week. Can someone help please. Thank you.

Which version of Audacity?
Which version of Windows?
How long is the audio selection?
What are the full settings that you are using in the Change Pitch effect?