I have a presentation file that has more than 130 slides. What is odd about this presentation is that while I cannot add new slides, I can duplicate existing slides. So I can still sort of add a new slide by first duplicating an existing slide and then wiping its content, but I do not like the extra hassle.

I do not have this problem with a newly created presentation file. 

Any idea why this is happening? I'd appreciate your help because I will need to do extensive editing to this particular slide set. Thanks in advance!