the stems on my yellow fowers on my tomato plants break off about 1/2 inch above the flower once it

I have several heritage varieties of tomatoes growing here in Calgary Alberta , and in a green house that  I usually keep the ends open on.

I transplanted them at 6 -8 inches tall ,and now they climbed 4 feet or so , and have had yellow flowers for over a month now , but very few tomatoes.

I just was inspecting the dying flowers , and the stem is breaking off about a half inch up from the flowers.

There seems to be a small bump at the point where the break later occurs, it acts like a knee, and bends downward and all on it's own breaks off.

I experimented with some that were ready to break, and the stem has a whitish color inside it ,which appears to be very much alive .

I have Black Krim, Chocolate strpe and Navajo purple , the Black krim is affected the least but still doing the same thing , just on less tomatos.

There is no sign of bugs knawing away , just a clean break like cut with a knife.

We have a hot summer this year and I used quite a bit of water in the green house , also in there my beets are growing like crazy ,and my green onions too, Even my cucumbers are doing well, just the tomatoes are a problem. .
Any help will be appreciated , we will be getting snow here in 2 months or less.
Neal

Maybe a pollination issue, since they seem otherwise healthy. Try vibrating the flowers or plants, or assessing your pollinator situation.

Edit: the flowers generally fall off at the joint if the environment is stressful or they are not pollinated.  Did you try the things suggested here with no luck?
https://permies.com/t/89549/tomatoes-loads-yellow-flowers-refuse

Thanks for the edit, I didn't know they broke off if not pollinated. I have shaken them and used q-tips and later a small paint brush , that maybe why a few are growing. I'm going back out with my paintbrush.

I'm running out of growing season, probably snowing in 2 months , but at least I use any green ones for salsa .
Thanks for the info.

When you go out there, do you see bees hanging around and in your flowers?

I haven't tried it,  but check out this guy's trick, it might help. He puts out a lot of good garden how-tos.

Increasing pollination by vibrating the flowers with your hands.

Another trick for green tomatoes in snowy climates is this: