Need help with Yugoslavia/Bulgaria Surrender

[Weasel]

I have read the manual (19.2) and checked the "what's new" and according to the manual these two countries should surrender and kick the Germans out as soon as a Soviet unit is adjacent to their border, in particular hex x42,y120.  For turns now I have had supplied units sitting there yet the Germans are still moving and drawing supplies through these two, is there something I am missing to get them to give up?

The one stipulation is "in supply" which my units are.

Does the unit have to be Soviet or can it be Romanian (I have had both sitting on the border).

Hopefully someone can help me on this...very frustrating.