This card can only be Ritual Summoned with the Ritual Spell Card, "Ritual of Grace". When this card is Ritual Summoned, select a number of face-up cards on the field (except this card), up to the number of monsters you used for the Ritual Summon. While any of the selected cards are face-up on the field, this card cannot be destroyed by card effects.
Only reason I can think of is effect negated, the card(s) you targeted with Northwenko's effect were gone, or you summoned her with Fullfillment of the Contract or Ritual Foregone so Northwenko couldn't activate her effect.
How are you trying to summon it? If you use Sprite's Blessing rather than Ritual of Grace, remember that you must offer EXACTLY the amount of levels: Sprite's Blessing doesn't allow you to go over, unlike most ritual cards such as Ritual of Grace.
It always worked for me well. I guess you don't offer enough tributes (their stars are important). Either way currently the only useful ritual monster is Relinquished...
I understand the effects can't destroy her, but they still affect her? For example can one turn her to defense position with "stop attack" or a Sphere Kuriboh, and then just destroy her? And how would an "interdimensional matter transporter" work with her? Can she select cards again, or the selection remains?
Destroying Umi is a cost, costs of cards happen BEFORE the actual effect, and happen even if the effect is negated somehow.
Deadalus declares effect -> pays cost (destroy umi) -> you can then chain something to his effect -> he then destroys the field.
Im 99% that the interaction above is correct, u can test it with a friend ingame too tho.