Garmin Fenix 6s Altimeter problem

My Phoenix 6s has been having problems with altimetry for some time now.
I calibrate the altimeter by gps, but after 2 or 3 days it's already out of calibration.
I have to keep calibrating the altimeter all the time and that's not normal.
Does anyone know how to solve this problem without having to calibrate the gps every day?

Thanks in advanced
Bruno