Robot combination in isaac sim

I created a car and a parallel robot in isaac sim separately and successful.But I cant combine them into a single usd file successful.I import a parallel robot file into the car ,they can move separatly,but when i build a joint between them,the parallel robot can not move?How can i fix this problem?this is the file i made and this is the picture. (871.1 KB)

I had the same problem, and it was still unsolved.