Is the magnesium turnings (like chunks the size of grains of rice or so) or is it really powder? If it's the latter, you may be in for a bad time, since the higher surface area means that more of it is coated with oxide and the small particle size means it's hard to chip off the surface by spinning it with a stirrer.
However, 99% of the time the problem is not with the metal, but with the solvent you are using. How have you dried it? If nothing else, you should be letting it stand over molecular sieves for a time before use.
Remember, anything you do to activate the magnesium should be done under inert gas the whole time. Argon is best as nitrogen will react with fresh Mg surfaces a little bit.
Are you adding iodine to the reaction as a catalyst and indicator? That may help. Otherwise, you can try to etch the magnesium in the reaction by adding a small fraction of dibromoethane or even liquid bromine and letting it react before adding your dibromobenzene.
I am sure that I have made this exact same Grignard before and that it gave me no trouble, so definitely check all your reagents.