Cobb's totem is backwards.
I got this from an article in Psychology Today.
The purpose of the totem is to determine whether you are in someone else's dream. For this to work, the totem must:
1) Be a recognizable object with expected properties (i.e., a die which would be expected to land on a random face).
2) The properties of the object must be modified by the owner in a manner known only to themselves (i.e., the die is loaded so that it always lands with the three face showing).
3) No one other than the owner may be allowed to handle to totem.
As a consequence, the totem will have a secret, nonstandard behavior in waking reality, but the standard, idealized behavior for that object in a dream.
Cobb's totem violates these properties. His totem is taken from his wife, has a nonstandard behavior (perpertual spinning) to indicate dreaming and has a standard behavior (the top loses inertia and falls over) to indicate waking reality. As a result, the totem doesn't actually test anything.
Cobb cannot realize he is stuck in a dream because his means for testing whether he is dreaming or not is false and reenforces a false consciousness.