Hello.
I have made the rig for suzanne and the right ear deforms properly but for some reason, the left ear does not. Can anyone please take a look?
Edit: I solved it by applying mirror modifier to monkey head but I still don't know how this problem occurred in the first place.
Here's the blend file
In your file, Suzanne's right ear (green arrow) is also being affected by the head bone (blue arrow).
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.