Just one other theory,...not sure on the North Star but some cherries have a off year.

Ditto for my North Star too. I have 4 sour cherries and
some did not bloom heavily this year. My North Star did
but very low fruit set. I was sorely disappointed as it has
bore well in years past. Plenty of bees/insects working the
flowers so it was not a pollination issue.

I have Stark's Surecrop cherry too. IT bore great last year but has some winter die-back this year. Not as many flowers as last year.

Hooray for my Mesabi cherry. It is just blooming and very heavy with flowers. I hope it makes up for my North Star.

I have Balaton, North Star,Sure Crop and Mesabi tart cherry in my orchard. Balaton is the best for fresh eating as very dark purple fruit and the sweetest tart cherry I have ever tasted.
My daughter will eat them off the tree but not the other tart
cherry varieties. Only bad news on Balaton is that in youth it is slow to come into bearing and does not yield as heavily as some of the others.

My North Star is perhaps my favorite as very heavy bearing
and I like the fact that I can easily keep it trimmed to 8' in height so easy to cover with netting when the fruit ripens.
My best yield on my North Star was 28 quarts (measured after pitting). Sadly it now is full of bacterial blight the past
3 years and I cannot get it under control. I tried copper and
had no luck.

I also have Surecrop from Stark Nursery. Appears to be
an amarelle type as has yellow flesh and a hot pink skin color. Very juicy and cooks up nice. It was very slow to
bear as waited 5 years for the first crop.

My Mesabi was an accident. It got shipped to the nursery I
work at by mistake so it was free for the taking. It is a young tree and growing well. Picked the first crop off it last year and bloomed well this year. The flavor,yield and hardiness are fine. I have not had it long enough to comment further on it.

A friend in Minnesota grows Mesabi and tells me the only issue for him is that the fruit is very thin walled (fleshed) compared to other tart cherries. Another friend who grows tart cherries (5 varieties for his U-pick orchard) tried it and told me the same.

I don't mind since the tree was free and I mix them with my
other varieties anyway after pitting.

The BEST yielding tart cherry I have come across is Sweet
Cherry Pie (TM) now released thru Bailey Nursery in Minnesota. I sampled it 23 years ago while visiting Bill Eubank (his selection) near River Falls WI. Bears heavily at a young age with fantastic yields.

A friend trialed this cherry at his orchard and found it usually out yielded all the other commercial varieties he
grows in tart cherries.

It is officially out on the market this year. Since you are in Minnesota you should be able to get one at any garden
center that carries fruit trees from Bailey Nursery.