Both employees and leadership suffer when there's a lack of trust at your business. Start building trust through the Employee Management Cycle, which will help keep everyone on the same page and prompt you to re-evaluate as needed.
You've got four options when you just can't get your employees to do something you've asked of them... and some of these options are better than others. Read HR Guy's take on how to proceed.
When your employees question your policies or act in unexpected ways, it's best to ask "Why?" before simply reacting or saying "Because I said so." This post explains the reason.
What you allow one employee to do, expect everyone to do. Here are a few tips for setting and maintaining expectations so one "bad apple" won't spread rot across your entire Brand.

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The HR Guy – HR Support for Service Businesses