Apparently a bug in PowerPoint 2013 and 2016 (not in PowerPoint 2010):
1. Type some text...,
2. Press SHIFT and ENTER (soft return) twice,
3. Insert some equation
4. again press SHIFT and ENTER (soft return) twice,
and you cannot insert another equation!! You can type, but you cannot see it.
For example:
- original presentation created with PowerPoint 2010
- presentation opened with PowerPoint 2010 with all slides saved as pictures
- presentation opened with PowerPoint 2013/2016 with all slides saved as pictures
