Some vertices of model imported from Blender do not move when animating

I made a model in Blender with a rig, and painted the weights myself. This is not a humanoid model, but a bird-like enemy which flaps its wings. The problem is that when I import this model in Unity, some vertices do not move at all when I move the bones! This problem does not appear in Blender so I am not sure why it happens. Any help please?

Confirm the vertex skinning influence in blender is 4 bones or less per vertex. If it is more than 4 errors will no doubt occour in engine.

I changed the quality in Skinned Mesh Renderer to 4 bones and it worked!