Make ALL grib files content visible in ZyGrib

[myocean]

I am struggling since long with the combination of Airmail used to download grib files and ZyGrib used to display them.
It appears that the same file, opened directly in Airmail (which starts ViewFax to do this) displays more data (e.g. cloud cover and precipitation) than ZyGrib (as said with the SAME file).
If I open the Airmail downloaded file in Zygrib, a number of relevant options to display, e.g. in the "Weather map" menu, are just greyed out - not accessible.
However - ZyGrib CAN handle such data, but only if it is allowed to download the data by itself from the server, with an internet connection?
I would like to benefit from the nice presentation of data in ZyGrib anyway. Is there a solution?

I am using ZyGrib 6.1.4 for Mac.