When you use acetonitrile I think that it is being converted to another species which then does the epoxidation.
Edit:
Here we are. The species doing the epoxidation is a peroxyimidic acid formed from the nitrile. See Part 3 - Discussion here:
http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?rxntypeid=110&prep=CV7P0126 (http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?rxntypeid=110&prep=CV7P0126)