PowerPoint formats changing for no reason

I have 11 power point presentations that I have been using to deliver training for a number of months. Two week ago I copied them to a thumb drive for a colleague to use which he has done for the successfully until last night.

All of sudden the formatting on random slides has changed with some text boxes changing size and when I checked my copies today the same thing has happened but again on completely random slides.

We are not using the same computer so there is no common thread there either.

any assistance would be appreciated