Why did one message on kik stay on bold d? Did they not receive my message?

So I sent a message to a friend and it stayed at status d. Never got a response for four days. Messaged them again and they responded. Though the message that they never replied to still says d next to it. Which I know that means it wasn’t read. How is that possible when the messages after it was read?
Why did one message on kik stay on bold d? Did they not receive my message?
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