I assume you mean your coolant reservoir, not your screen-wash bottle.
You either have a leak, an airlock, or maybe the thermostat or the head gasket is on the way out.
Is it overheating at all?
Are there any signs of liquid on the ground under the car after it has been parked for a while.
Any smells of damp, or damp mats, inside the car (the heater matrix could be leaking).
Is the heater working properly?
When was the coolant last flushed out and renewed?
ChicksFan
November 2011