Elven Cloaks: magical or not?

[Gorwingel]

This is one thing I was wondering. In the scene where Frodo runs down the rocky hillside to where Sam has landed and then Frodo covers the two of them with his cloak so they are not seen by the Easterlings they seem to totally dissapear. But during the scene where Merry and Pippin are crawling away from the orcs (and they are presumably wearing the same cloaks, unless Galadriel gave Frodo an extra special cloak) they are still seen, even though they are covered by the cloaks. Are they not wearing them correctly? Because you would think that the orcs are unfriendly eyes, so they would probably not be seen? that is just something I have been thinking about for a while.