Many dream symbols out there in the world are totally unrelated but in many ways they can be related back to one another again in the dream world.
If you have a dream about a dull knife, then you are probably frustrated. You are frustrated because you have been using a knife and it has not been working. This knife, however, is a figurative one. A knife is known for two things, its ability to hurt someone or something, and its sharpness. Either you have purposefully been trying to hurt someone’s feelings recently or you have been trying to come off as smarter than you actually are for the purposes of impressing someone or something. Instead you’ve come off as pretty full of yourself and you are starting to make yourself look ridiculous because of it. Pro tip, don’t try hard to be something you’re not.
This is why you’re dreaming of the dull knife. Maybe you’re just too good of a person at heart to really hurt someone’s feelings or maybe you’re just not as smart as you wish you were. The good thing is, a true friend will accept you for who you are anyway. You don’t need to try to impress anyone that wouldn’t accept you for who you are anyway, so good riddance to them.