"Bad Quality" tag in designer

IN THE QUICKCLIENT MY TAGS ARE SHOWN AS GOOD BUT IN THE DESIGNER THEY ARE SHOWN AS BAD AND THE VALUE BECOMES NULL.
why does that happen?

{ Please don’t shout. }

The most common reason is an OPC server that doesn’t support subscriptions properly. Setting the tag group to OPC reads instead of subscribing is a work-around. (Better to fix the OPC server if possible, though.)