There used to be more items you could not swap out the enchantments on, like most dropped set items. However now it is rare to find something you can not apply a different Glyph too.
One thing to keep in mind, you can only apply certain enchantments to certain types of items. For example, armors can only take attribute enchantments. Jewelery can only take regen, cost reduction, power and shield type glyphs. Weapons have their own set too. So, you may not have the right kind of enchantment to apply to an item. Also, you may not have the correct level of item for what your trying to enchant.
About halfway down you can see what types you can apply to what items.
well, I think I know that... for instance I tried to enchand that ring with gliph of increasing magical harm...which some has on them....
and thx alot for tips!