Servo + Motors off 1 power supply

So I’m trying to control a servo and 3 motors through an arduino, powered by a single 9v power supply. They motors work fine and the servo works when the motors aren’t connected but when the motors are connected, the servo jitters loads and doesn’t turn. If I connect the servo to a separate power supply, it stops jittering but it still doesn’t turn.

What could be wrong?

Maybe there’s a problem with the schematic you didn’t show us, or the unspecified 9V supply.

Update: it does work if I use two separate power supplies.

I’m using two variable power supplies used for school practicals, both set to 9v. The servo is a 5v servo so I’m using a voltage regulator. I want both to run off one power supply but when I connect both up, the servo jitters a lot and won’t turn.