I would guess it is your fingering.

I find the low D Susato I have is trickier than a reg. D. With the reach it is often hard to feel the holes. I assume you know to cover 1, 2 , 4,5 holes (counting up from the bottom) with your finger between the 1st & second joints, not with the tips. Sound like you know that from the way you talk about it.

Second guess would be breath control

Has this whistle played ok in the past or is it a new whistle? Or you might have a bum whistle I guess

Sounds like you are just rusty, try playing slow & easy it maye be a combo of the two probs (Fingers & Breath) Good luck Nick