For a few weeks now we have found that PowerPoint does not load using the "add item" route. We get a grey rectangle on the schedule but it never gets filled with a thumbnail. This applies to .ppt and .pptx.
However we can load the PowerPoint into the presentations database and load it to the schedule from there. The reason we prefer not to use this route is that we have a weekly notices PowerPoint and need to remember to delete the old one before adding a new one - otherwise there is a danger of putting last weeks notices in the schedule!
We have also had a number of instances where a PowerPoint is happily rolling on the "Live" pane, the one where you can select which image to go live, but nothing is appearing on the output. Closing EW7 and reopening usually cures this.