Can a lipomas still grow in same area if a piece was left in?

Lipomas can regrow particularly if they were incompletely removed. I am not sure why being close to the knee muscle would affect removing a lipoma. I frequently remove lipomas from with in muscles although usually this would be under general anaesthetic.I am not sure what 'my leg goes to sleep' means. Compression on a nerve can cause numbness in the area that the nerve supplies with feeling. it would be most unusual, but not impossible, for a lipoma to compress a nerve and cause it to stop functioning. However, I would suspect that there is another explanation. I suggest you see a suitably qualified and experienced surgeon so they can examine you in person and give you an opinion.Best wishes.

I'm not sure what you mean by your leg going to sleep - do you mean parts go numb or weak? A lipoma or any other swelling could have such effects if pressing on a nerve, but generally they would have to be pretty large to have that kind of effect, and a different cause such as nerve entrapment (or perhaps post-surgical injury) is more likely. 
To answer your original question - yes absolutely, a lipoma can recur if parts are left behind, which is why treatment by liposuction has a higher recurrence rate than formal excision. However any residual lipoma tissue would generally continue to grow at around the same rate it had done before treatment - if that was very slow then you will probably never notice a recurrence.