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This is not uncommon with MX blues.
I have a Ducky with MX blues and the up arrow and X keys did not click at all.
I totally spammed the keys, the up arrow in particular, and after a week or so of that (programming and editing, repeatedly pressing the key instead of holding it down to move up in a file) it did start to click.
The X less so, although depending on what angle you hit the X you get a louder or softer click.
Apart from that, you'd need to replace the switch (open keyboard, void warranty, desolder old switch, solder new switch, close keyboard).