Sounds like a bad luck streak. Some things are just meant to be, like until recently, my Pegi knights refused to make a fear/terror test to charge. Failed 3 in a row in one game, they ran off the board and never touched a thing.
If you look at the math, they should pass the test 6 out of 10 rolls, but for me, they fail 9 out of 10. Until recently, I fired them and stopped bringing them for a while. Now they pass as often as they are suppose to

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The other thing you are running into is that I believe Black Knights are only weapon skill 3. Which means in both of your examples, the knights you hit are returning hits on a 3+ vs your 4+. even if you get a killing blow, the brets have a ward save to stop it on a 5+. The Dragon Princes get 2 attacks each where the Brets should have rank bonuses. The only thing worse for Black knights charging Brets in the front is getting charged by them.
The best thing VC can do against Brets is to try to gat DM of and hit them with a unit of infantry and try to get a side or rear charge off on them with the Black Knights. The Dragon Princes? Hard fight unless you get lucky. Then again, they do cost a lot more than Black Knights to field. Blood Knights on the other hand would make quick work of them if they got the charge.