[Mark B]

Joining NATO requires unanimous consent of NATO members, and getting unanimous consent can be damned difficult.

Germany has indicated that it won't approve Ukraine joining NATO while it's at war with Russia, and Turkey as yet does not support Ukraine joining NATO, while at the same time Turkey is trying to maintain friendly relations with Russia while also supporting Ukraine.

Turkey is also holding up Sweden's membership in NATO over an accusation that Sweden is harboring members of the Kurdistan Workers’ Party (PKK) and the Democratic Union Party (PYD).