I'm not sure that's a problem with communism so much as it is a problem with wage labor in general. Even salaried employees have no financial incentive to work any harder than they need to in order to keep from being fired, unless they're on the track toward a managerial position.Communism has been tried and it was found that people lost the motivation to work because the reward and compensation was taken away from them. And since no one will work for someone else, without the reward, people will not invest hard work.
I would think the more reasonable sounding problem with communism is the question of why someone would become a doctor rather than a fry cook when both make the same amount of money. I don't know that there was ever a study conducted on how many people with the potential to become doctors instead opted for fry cook, but there is something to be said for the prestige and satisfaction that come with doing a more difficult and rewarding job, even if you are not paid fairly. Otherwise, why would anyone in the US ever become a professor? And, in fact, we still have that problem under the American system, whereby certain jobs - fireman, schoolteacher, social worker - are essential and require a great deal of skill and hard work, but don't attract the brightest and most qualified among us because the pay is quite low.
So each system has its own respective motivation problems. Under communism, it could be said that one might opt for the easier job, while under capitalism, one might opt for the job that pays more. In neither case is one opting for the job more important or essential to the overall health of the society.