Probably because I don't think you can turn the p-channels off with 12v on the source and 4.5 on the gate. So they were always on and then at some point you also turned on the n-channels and the electrons hit warp 9.
Even when you start using logic or the Arduino you'll have to level shift the p-channel's gate signal with another transistor AFIAK. Or get the special ones with the internal charge pump.
BTW the above is why I counseled against using 4 pins from an Arduino with no logic, it's only a matter of time before things go poof.
Rob