undergraduate bromination lab troubleshooting

My organic 1 course runs a bromination of trans-cinnamic acid lab. The bromine was previously dissolved in DCM, as was the trans-cinnamic acid and we never had a problem. We recently changed from DCM to glacial acetic acid, and the reaction is not working. The color from the bromine dissipates slightly, but not all the way. Ideas as to what might be wrong?