To my experience, the Import building only levels up when the two cargo types are imported. When importing only one of the two, it does not reach the threshold to upgrade. Maybe that is also the problem you have?

I have to make it clearer, I just observed it a little more:
I did not mean the "level", but the "output" - I use german localization, but the bar below the level which tells how much is sent out.
So, the Import Building by default is on level 1 with 300 production from which 100 is the "transit goods".
When I create a transport chain to the Export Building the "transit goods" are transported by a rate of 100, which is the output of the Import Building. So well for now.
Then the Export Building levels up twice, but still, the output of the Import Building stays at 100, even after I tripled the trains to have a higher possibly rate.
It never catches up and sticks at 100.

I got it - it's just as you said - it's necessary to at least 2 of the 3 output goods to be sent out to fulfill the needs for upgrading the "Import Building".

I'm glad you figured it out. And thanks for the feedback that is works.