So I have 15 Daedric Artefacts needed for the achievement, and yet it didn't unlock I own the Ebony Blade, Ebony Mail, Oghma Infinium, Black Star, Mehrunes' Razor, Mace of Molag Bal, Dawnbreaker, Spellbreaker, Masque of Clavicus Vile, Ring of Namira, Skull of Corruption, Sanguine's Rose, Saviour's Hide, Volendrung and the Wabbajack.

Anyone have any ideas as to why it didn't unlock, and if it's possible to be able to get it to unlock?

It's a bugged achievement and has been for a long time. Seen multiple theories as to why, but the most concrete I've heard is that Dawnguard screws it up by making one of the artifacts not count. I have seen reports of it unlocking for some people after using the Oghma Infinium or picking up the Skeleton Key, even though that one isn't supposed to count, so you might give those a shot. Also, both artifacts in Ill Met By Moonlight (Hircine) count so one work around is to get both artifacts, although this also only sometimes works.

If you decide to try again then on the next go then get both the Ring of Hircine and Saviors Hide as this seems to help.

To get both artifacts-