For the song that will not play, close Studio One. Browse to that songs folder on your computer. Delete the Cache folder. Reopen Studio One and see if it will play.
If it will not play, open the Performance Monitor in that song, click on the "Show Devices" check box, then turn off all plug-ins. See if the song plays. If the song plays with the plug-ins turned off, reenable one plug at a time until you find the one that is causing the issue.
Please submit a ticket to support if you require more detailed assistance on this.
Here is a link to create a ticket: http://support.presonus.com/hc/en-us/requests/new