Hi William....welcome to the Forum
I can only think it is one of two things...
1. Pressure cap is defective, not holding pressure and letting air in and pressure out to allow the boiling point to be too low, hence losing coolant out of the reservoir.
2. Air lock somewhere up by the matrix. I used to have a tool when I owned a garage which vacuumed down the entire cooling system and then purged it with coolant under pressure, was sometimes the only way to get the bl**dy air out of the cooling system.
Sometimes, it won't necessarily leak coolant but you can be introducing air into the system via poor sealing joints or defective cap...worth checking before condemning head gasket, etc
Let us know how you get on with finding the fault
Cheers Trevor