Our workers are very responsible and can never disappear without a trace! Depending on how many workers you lack for a particular production, hiring one or even four workers with emeralds may not be enough. Spending emeralds doesn't guarantee that the production will be possible, it all depends on the number of workers available and the number of workers needed in this or that factory.   
You can also buy residential buildings for coins instead of hiring workers for emeralds. Each residential building increases your total number of workers permanently (unless you decide to sell the building later).