Should is an important word in the English language. It helps us express values, as in, "You should always be fair to others." It helps us give directions, "you should see a gas station on the right."
But if you catch yourself saying or thinking "should" about something factual, it's time for a cop-out check. Using "should" to describe a state of knowledge can be dangerous and you should immediately go into fact-gathering mode.
Remember, part of your job as a boss is to have a clear understanding of reality.
When you say, "He should know" find out if he really does know.
When you say, "She should understand" stop and find out if she really does.
There are lots of reasons that people who work for you might not know or understand the things you think they should. Determine reality so you can make the best choices about improving performance.
It's your job to make your expectations clear. You can't see inside a subordinate's head to discover their version of your expectations. The only way you'll know is when those expectations turn into behavior.
You can wait for their performance to tell you, which may be too late. Or you can ask and let their words tell you. That way, if you and your subordinate have differing understandings of your expectations, you've still got time to adjust.
So, the next time you're tempted to say "should" about something factual, take time to check the facts.