Storage coolant thermos problem

I have a Prius 2004. I bought it last year. I have a problem (I think) with my coolant storage thermos. When I bought it,

the pump was working (bringing back the hot coolant from the thermos to the engine) even after 24 hours and with cold

temperature (-10 Celsius). Now it does not start even after 8 hours at -5 Celsius. I though it was a problem with the pump

that was a bit noisy, but even after I changed it a couple of week ago, the problem remains. The pump is working fine when

i stop the car and it is bringing the coolant into the thermos. Anyone having this problem?

Thanks!