An example from TED:
Those conversations should be happening in a good marriage, not after it is broken.
And I have heard it many times so far, but I am not sure what exactly it means. Does it a more polite or uncertain way to say something that will happen?
In a different context, the phrase "X should be happening" would indicate an expectation rather than a surety.
Examples:
The guests should be arriving soon
This means something like: I have not confirmed the exact location of our guests, but I expect that they will arrive soon.
When is the water going to start boiling?
It should be happening now.
Again, this would mean: I haven't actually checked the pot, but (since it's been on the stove for a while) I expect that it's boiling.
On the other hand
Should can also indicate an obligation, or a recommendation, so X should be happening can also mean you are obligated to/it's recommended that you do X.
Examples:
You should be doing your homework
This might be what a parent would say to their child when they find the child playing video games before having finished their homework.
I manage two employees; how do I know if they're happy at work?
Conversations with your employees about job satisfaction should be happening regularly.
Here, the should expresses the sense of a recommendation; it's recommended that you regularly talk to your employees about job satisfaction.
So how do we know which sense applies in your case, or in any given case? Context.
Because the speaker says that this "should be happening in a good marriage" and "not after it's broken," it appears that he is making a recommendation. That is, this is what you should do if you want a good marriage.
As far as I can understand from the given point that expression means is supposed to be happening as just the way the thing are happening naturally.
