How do I report a technical problem related to converting from Banktivity to Quicken for Mac

The way to report a problem like this is to use the Report a Problem option on the Help menu in Quicken. Document the problem as fully as possible, and include any necessary screen shots or log files. you can post additional details in this thread, and then include a link to this thread in your problem report.

The key is that someone at Quicken needs to be able to replicate the problem and confirm it -- which is difficult dealing with other application data that they likely don't have much of, and which many people are reluctant to share with Quicken without that. But if their 'triage' people can't reproduce the problem, they likely won't investigate further. 

the other 'problem' with this method is that you won't hear back from Quicken, except in rare cases where they want to reach out for more information. So there's any way you'll know if they agree with your bug assessment and have placed it in their database for future action or if they couldn't replicate it and tossed it. And of course, even if they replicate it and put it in their database, there's no telling when it will get acted on. The developers have to prioritize the importance of all bugs reported, as well as enhancement projects, to decide where to spend their time. At one extreme, if they find it's an easy thing to fix, it might be fixed in the next maintenance release; at the other extreme, if they determine it's a more involved issue and one which affects a small portion of the user community, it might be slotted way down the roadmap for action well into the future. And again, we as users have no idea where various issues are on their action list.

I'm not trying to discourage you; to the contrary, I encourage you to report the problem you've found! I'm just trying to give you a realistic picture of the path ahead when you report such an issue.

In this case, I'd imagine a main issue will be whether there's something in the QIF export from Banktivity that is wrong, or just different enough to not work with the current Quicken import code, and whether they can work around it. Can you isolate a few transactions from one of the small cap stocks with didn't import and compare them to a stock with did import to see a difference? It could be a wrong character, or code in the tag-value pairs that comprise a QIF file. (Lack of consistency is why Quicken moved away from QIF files many years ago, and mostly doesn't support them in the current product.)