Eh - I know from experience in my family that just because you speak to children in one language doesn't mean they'll reply to you back in it.
My youngest sister refused to speak English to my father after she realised nobody else understood her when she started kindergarten; occasionally she would even claim not to understand English. But when she was in America as an au pair with her cousin, she was a little satisfied to find that her English was better than her cousin's -- something must have stuck -- and when she had children, she raised them in English. (Recently with some German mixed in.)
And some of my nieces and nephews speak English fluently, while others will understand it when spoken to but will reply in German.
I don't think the difference is in how much effort my sisters put in, but more about the children's personalities.
As such, I'm grateful that my daughter speaks English to me, because I realise that it's not a natural and necessary consequence of the offer of bilingualism.
So I would like to ask you to paid â phoeni (don't worry): even just understanding another language is a lot more than a monolingual has, and I don't think this level reflects badly on you as "teacher" at all!