Is your system sealed or open vented?

IF sealed, the only causes I can think of are
(a) airlocks in pipes prevent all the air escaping when
you fill.
Partial cu fill, run pump at full power, vent, refill.

(b) The fill water contains a lot of dissolved gas.
Run hot, vent & refill while still hot.

If open, you might be over- or under-pumping. Only
cure is to alter pipework, but a big dose of anti-rust
might put off the evil day...