DIAGNOSIS That's usually a sign of a clog, either in the internal tubing or, more commonly, at the shower head. Mineral scale or coffee oils are blocking the flow.
FIX 1) Run a descaling cycle with a proper liquid descaler (like Breville's own). Follow the machine's descale program. 2) After descaling, take off the shower head (it just twists off counter-clockwise). Soak it in warm, soapy water and use a toothpick to gently clear any debris from the tiny holes.
IF THAT DIDN'T WORK The internal pump might have a piece of scale stuck in it. Try running several cycles with just clean, hot water (using the 'Gold' setting) to try and flush it through.
HEADS UP Never use vinegar. Breville specifically advises against it for this model as it can damage the thermal carafe lining and internal components.