Any welders on the board?

I have a question about mig welds not penetrating.
I'm using .030 wire to weld body panels,sometimes the it will weld good(plug welds)and penetrate other times it won't!
It will just fill in the hole and as soon as i touch the weld to see if it held it brakes!
The heat is on 4 and the wire speed is up about half way,i can't remember where it's set right now.
Any tips or suggestions would be appreciated. :)