Formatting problems

I will save a presentation in powerpoint, and the next time i open the
presentation, the formatting is all screwed up. There are 5 masters for the
presentation, and sometimes if i apply a master to a page, all the other
pages hooked up to that master will change into a weird format. It is only
half the presentation, but this is the 4th time i have had to change the
presentation back to the correct format. It is nothing big, it just knocks
out the bullets and changes the color of text, but it is a presentation that
is constantly having info added to it. I don't want to have to change the
text on every page every time i change the presentation. Please help.