macbook freezes in power point presenter view

I have a few faculty members who use MacBooks and PowerPoint. They can mirror Power Point just fine, but once they start their show in Presenter View their Macbooks freeze. They're both connected to AMX DVX switchers, but my Dell works fine using VGA or HDMI. Anyone know what the Macs are doing to themselves which sends them into fits? Does anyone have a solution? Both are running El Capitan, one had no problems until he upgraded.