My iMessages aren t sending?

I have an iPod Touch 5th generation and my boyfriend has an iPhone 4. We use iMessage to message each other (it s sent through messages, not texts, because I m using an iPod and my boyfriend ran out of data so he uses wifi.) But I texted him the other morning and it said delivered but he never replied. So I sent him another one a few hours later and it took about 4 hours for it to say delivered. There was no reply so the next day I messaged him again and it delivered and he immediately got it, and he told me he didn t get any messages from me the day before. We were able to message for a few minutes until I last messaged him, and it delivered, and he didn t get it again. What s the deal?

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