Trying to "make everyone happy" is not a manager's responsibility. Don't even try because you'll become a door mat in short order.

Each employee has to make their own decision if they are happy doing the job they do or not.

You cannot solve everyones' problems. At best, a good manager creates an atmosphere in which their employees feel as though they can express their concerns without fear of punishment or recrimination. Sometimes people just want to get something off their chests. It's when they can't express themselves that frustration begins to ferment into disloyalty.

If employees feel as though they can come to you with a problem, that's enough.
You may not be able to solve the problem. It may not even be yours to solve in the first place. But if they feel as though you listened and heard them out and maybe offered some advice or suggestions your battle is half won.

Be yourself. But remember, plumbers are well paid because of what we are not willing to deal with ourselves. Your unhappy employees are free to wallow in misery or leave and try to find another job.