Seals not working? (AST)

Just a quick question, I'm confused about the new Astrologian seals, because sometimes they activate and appear on my arcana gauge, and other times they don't... I've hypothesised a few reasons why like "both the AST and target must be in combat" and such but sometimes even when that requirement has been met the seal doesn't appear on my gauge... Could anybody please explain to my in-depth how the seals work and why they sometimes don't appear on my gauge?

Thanks!

you must be in combat for them to work so no pre setting it for the divination pull

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Originally Posted by EmilyRoze

you must be in combat for them to work so no pre setting it for the divination pull

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I see, thank you!