This will sound silly, but it's actually a question many people forget to ask when things like this happen.

Just as if you had lost your keys you would ask yourself "Where did I put them last time?".

In this situation when you have had lucid dreams but suddenly lost the ability to induce them again you can ask "What did I do last time that I don't do now?"

This however has a much more complex answer, because those days she were perhaps less stressed, went to bed at earlier bedtimes, or something else only she can answer the question.
Although those two I took as examples are the most common problem I see people make and as Linkzelda said it can aslo be counterproductive to be too concerned with results.

So yeah in short try to find out what changed.
I wish both her and you sweet dreams