There are several possible explanations. I'll try to list a few:
(1) The other slides have things that cover up the logo. Whatever is on
the master slide is always behind whatever is on the slides themselves.
(2) You have a title master, and the logo is either on the title master
and not the slide master or vice versa. By default, there is no title
master, but you can add one. The title master is for putting things on
title slides and not on regular slides or vice versa.
(3) You have multiple masters. PowerPoint 2002 and above allow for
multiple masters, so you can use different master slides for different
regular slides. What you put on a master slide will only affect the
slides associated with that master.
(4) You have chosen to Omit Background graphics on some of your slides.
This is found in the Format menu under Background. There is a check box
for this and the option to apply to all.
I'm probably forgetting something, but these are the likely reasons why
something on a master slide would not show up on some slides.
--David
--
David M. Marcovitz
Microsoft PowerPoint MVP
Director of Graduate Programs in Educational Technology
Loyola College in Maryland
Author of _Powerful PowerPoint for Educators_
http://www.PowerfulPowerPoint.com/