You might be able to temporarily increase the opening height by wedging some cork under the low C# key cup arm - or have a glamorous assistant gently lift the cup up as you play the note.
If that makes a difference then that's the problem - and you can have the mechanism tweaked.
However, I suspect it's more a case of getting your chops in. As respected as the MkVI is, it's still an old horn - and it's quirky in places - which means that there are some notes that you have to work at.
You could try the old trick of dropping a couple of wine corks down the bell. If this improves the subtone response then there's a good chance you've got a small leak somewhere, or your breath support is letting you down.
Regards,