All the matrix is is a heat exchanger. If it isn't working then either the water still isn't getting through, or the air isn't. I would suggest disconnecting the two hoses and flushing the matrix backwards withe mains water pressure. If it is blocked and doesn't clear, then it is a new core. If it is blocked but clears, then it is sorted. If it isn't blocked, then either the air side is blocked by leaves or the water flow is poor( pump or thermostat?).