Something I've learned both from my brief stint studying linguistics and anecdotally through friends is that, when it comes to raising kids in a bilingual household, it's very difficult to persuade children to actually speak the non-dominant language.
For example, if a household consists of a mom who is a native Spanish-speaker, a dad who is a native English-speaker and kids who are being raised to speak both - but who attend an English-language school in an English-speaking country - they will very often grow up understanding Spanish perfectly but speaking it like a second-language learner, as opposed to a bilingual person.
There's not much a kid can do to get his mom to stop speaking Spanish to him or her, but the kid certainly can refuse to reply in Spanish. It happens often and it makes sense. Even in this increasingly flat world in which mastering multiple languages is proving to be a necessity, it's dangerous for a kid to be marked as "other" by classmates. At first, the kids are fighting back against the parents in an effort to be seen as "normal," but later in life, when they realize that speaking Spanish or Mandarin or whatever fluently would be an enormous asset personally and professionally, it's sometimes too late to become 100 percent fluent. Or it's too frustrating. Think about the difference between making mistakes and being corrected as a little kid naturally developing language skills vs. being corrected by your LAME MOM as a know-it-all 18-year-old. It's super easy to decide to give up in the second scenario.
It's not impossible by any means to improve dramatically after the magical language-acquisition window of birth to 12 or 13 years old, but it requires effort, motivation and willingness to make mistakes. Just speak. Let mom correct you. Deal.
So ... why do I still respond to Jesús in English 75 percent of the time, even when I initiate conversations in Spanish?
(That said, my Spanish is probably better than ever. But - here we go again - I'll never be bilingual living outside of the Spanish-speaking world. And, no, California is not part of the mundo hispanoparlante.)