Yep – @spikeyxxx is onto it.
The right ear will have weights from the left side.
If you are using the mirror modifier, you need to make sure the the .R vert group exists so the modifier has something to mirror the weights into.
It cannot create this group for you.
You also should make sure the mirror modifier is above the armature modifier, other wise it can’t see that data too.
Hopefully that explains one of the possible causes in your situation.
