I can think of two possibilities. If you squeeze that hose for a few seconds, then you start to get hot air from the heater, that indicates a lack of coolant circulation due to the level is low. Fill the reservoir and look for a leak. You should get better circulation when engine speed is raised.
If you have to squeeze the hose closed for many minutes, that suggests the thermostat is sticking open and not giving the coolant time to get heated up in the engine. Proof of that would be found by feeling the hoses after the engine has had time to warm up. They should be too hot to hold onto for very long. You can also tell by the temperature gauge on the dash.