Collision detection problems with plane + 2D Sprites

I am experimenting with making a first person shooter with 2D sprites as enemies as the good old Doom days.

The enemy is a plane mesh with a transparent diffuse so the png sprite looks right. It is using a mesh collider.

I am having a problem with the collision between bullet (sphere) and enemy (plane). The problem is that the bullet just passes through it without activating the trigger event that tells the bullet to destroy itself (tags and stuff are OK and the object is marked as a trigger)

On some random occasions the trigger event works, but that is only if I shoot at the edges of the mesh and it doesn't work 100% of the time.

Any help on this issue will be greatly appreciated.

Thanks in advance :D