Staff Happiness plays a big role in how fast they work and how motivated they are. Too much work is bad, so is not enough work. Throwing Money at them doesn't fix the Problem in the long term. Check the Settings in the Staff Room to see if you can make your Staff more happy. If certain Shops are too busy, consider building another one of that Type close to it. Managing staff can be tricky, but if you keep them happy they shouldn't give you any issue.