it never has it can fire if your weapons are in or out. so its not DP that is making your cannons not fire it might be your power settings.
That has caught me out a few times, recently. Lasers and the like use a lot of power and if you don't keep your pips high in weapons the capacitor discharges and none of your weapons work. I noticed this in the recent Beta and now in Live.
Next time it happens. (Assuming your load out has thermal and kinetic weapons.)
Try putting 4 pips to weapons, take your finger off the trigger (not long, until there is some power showing on the lasers one or two seconds). Then try your cannons.