There was time when my kids (two French/American daughters) were simply not motivated to speak, read or write English. It seemed to be a combination of they didn’t want to be different, there were no rewards (none of their friends spoke it) and it was (in the words of my elder) “hard”.
The situation with the school a few years back was also not very helpful. I was scolded by one of my daughter’s elementary school teacher for speaking English at home. She assured me that they would never achieve a good level in French as long as I was there to confuse them. In all fairness, I recognize that it is not the job of the French public school system to teach my children English or to support me in my goal to have bi-lingual kids - their objective is to teach the national language and form French citizens.
So, I figured I had to tackle this one on my own. First step was getting some very good books about second-language learning by children (I highly recommend Kenji Hakuta). Second step was to stop asking advice from the school. Third step was OPOL (One Parent, One Language) and I enforced it. I refused to answer any request by the girls in French and I made a rule that all English or French books and films had to be read and viewed in the original language. It was not fun at the beginning. I was, apparently, the meanest nastiest Mom in the world because I wouldn’t let them read Harry Potter in French. There were also a lot of tears when it came time to write Thank-you notes in English for birthday and Christmas presents.
Seems to have worked out alright. My elder has a very good TOEFL score and she got a good grade at the Bac. The younger was able to get into an international school. I’d say that English is still the weaker language but not by much.