I had a situation where on my paladin I'm leveling up I bought the glyph to increase my Blessing of Might spell from the default 10 minutes to 30 minutes. Then I leveled and got a new rank of it, and the duration went back to the old 10 minutes even though I still had the glyph equipped.
I opened a GM ticket and they responded that this is a known bug they will fix in a coming patch. They were not clear if this would be before or after the Wrath release. So basically, if you're going to get more ranks of a spell, don't buy glyphs that affect that spell. Of course if it's not fixed before Wrath, we are all going to be leveling to 80 and will lose the affects of the glyphs we have. Just a heads-up for those of you thinking of investing in some of the more expensive glyphs.