USCIS asking for more evidence?

Hi, my spouse and I are trying to get our conditional status off of my status. We filed form I-751 and gave them an evidence of that we have been living since we got married, car insurance with our name in it, electricity bills, and our taxes, W-2 forms,etc. Both have our names in it.

After 2 months they sent me a form that says they were extending my stay for another year. Which I thought was a good thing.
After that, 3 months later they sent my spouse a mail about not meeting the requirements of proving that I and my spouse have this whole marriage thing as true, they have told us to provide more evidence that this whole thing was true.

The mail they sent us says that we need to send them more evidence (mortgage that have our names in it, kids birth certificates, combined bank accounts) and they are giving us until August 18 to provide those evidence.

My wife and I are both students, ages 22, we don't have kids we moved a lot, we thought that we have provided enough but I guess not, we don't have what they are asking. What should be my next step be? I talked to my spouse and we both agreed to write a letter to the UsCIs about our situation. Is this the right thing to do? Any suggestions?

Thank you for your time.

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