Why am I not able to retrieve iMessages prior to February 16th?

I

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I wanted to go and copy a conversation from January and I can only go as far back as Feb. 16 th. I didn't delete anything....why can't I retrieve further back ?
 
I

iMore Question

I have an Iphone 4s and I can only retrieve my messages back to Feb 16 :(

I wanted to go and copy a conversation from January and I can only go as far back as Feb. 16 th. I didn't delete anything....why can't I retrieve further back ? It is imessage from 1 iphone4s to another ipone4s ...