"She was criticized by her fellow lawyers not because she was not hardworking"
means that the reason she was criticized was for something other than her work ethic. It can be interpreted in multiple ways, unfortunately:
- (1) she was "not not hardworking", meaning that she was either hardworking or neutral (not considered hardworking, and not not considered lazy either).
- (2) she was "not hardworking" but this was not the reason she was criticized.
The bottom line is that with sentences like these can be ambiguous and if you want to be crystal clear to someone, it's better to avoid this issue altogether by writing something like:
- "She was criticized despite being a hard worker, because she failed to bring the expected number of cases to trial" if the intention is to say that she was hardworking, and
- "The fact that she was not hardworking was not the reason why she was criticized. She was criticized more for her failure to bring the expected number of cases to trial" if the intention is to convey that she was not hardworking.