How to intervene with a student struggling in the major?
I can tell for you from troubled student student perspective. I have some friends with such kind of problems here at Brazil in Usp and I think the our academic culture have similarities (including eventualy diagnostic tests), maybe my experience can help you to think the approach. 

One or another of my troubled colleages have to work a part time job to pay the living on the city where the university resides, some have some psychologic problems triggered by pression of university or self induced pression by a lot of expectatives and a no neglegible number have the three causalities combined in diferent proportions. And I think the culture of my generation (near 22-28 years) agravates that scenario, where people - having growed up with video games, internet and shinny especial effects every where - think to live fast and expect constant and fast feedbacks , what make one guy or another throw things to the air when the progress looks like very slow, even when need to be because the situation - the expected reality and the experience don't match

There are educational sites, like coursera.org, duolingo.com , brilliant.org and even the stackexchange that have adapted for some expectations of new users with fast feedbacks. In the subject, maybe electing some 'landmarks' to people don't resign in the middle of the way can be helpfull if is this kind of problem that ocuring. This can be real, I know a lot of strange examples, but the most intriging for me is the case one friend of mine whom had completed all calculus courses in coursera.org and got stucked for 3 years in calculus 2 ( and now he works as data scientist in a good enterprise )

If the problem is triggered by some kind of anxiosity, I think that if the students can feel they can count on you for a little and sincere talk in the final of some lecture or another, without being harsly judged - because probably they had heard hints for changing the major or something like that , a kind of talk that could be uneasy and even kills the trust - I just can think a good outcome being constructed along the semester  

Well, I hope this can help