Product: Bank account or service

Dear Sir/ Madam, I am writing to you today about an issue I had with Citibank ; and more specifically, a Citigold checking account that I have opened with them XX/XX/XXXX. At the time of opening the account, I have used a promotion that should have awarded me XXXX XXXX XXXX miles after meeting certain requirements. After opening the account, I have confirmed with Citibank customer service in writing that my account was successfully enrolled in the promotion and that I am eligible for it. The following is an excerpt from the answer I received on XXXX/XXXX/XXXX : " Thank you for choosing Citibank for your financial needs. I can confirm that you opened your account with the promotion XXXX for XXXX AA miles. '' On the next day, I have had another interaction with Citibank and provided them with my mileage account number. Again, I have received an answer that implicitly confirms that they see I am enrolled and eligible in the promotion. They have even cited the requirements that I need to meet in order for the miles to be deposited in my mileage account. The following is a part of the message I did receive : " I will be more than happy to assist you with the XXXX AA miles promotion for checking account ending in XXXX. Thank you for providing your AA account number. Our records show promotion code XXXX is applied to your checking account. Listed below are the requirements you must complete to qualify for the offer : Open a new Regular Checking account in the Citigold Account Package by XXXX/XXXX/XXXX. Within 60 days after account opening : complete Qualifying Debit Card Purchases of {$1000.00} using the Citibank Debit Card which must be posted to the new checking account. And 1 qualifying bill payment must be posted to the new checking account each month for XXXX consecutive calendar months. The AAdvantage bonus miles will be credited by Citibank to your AAdvantage account within 90 days from the end of the statement period in which you complete all offer requirements. ". 
Not only did they confirm the requirements but they also confirmed the deadline for the miles to be deposited which is fine so far as it is consistent with the timeframe they provided when I opened the account. On XXXX/XXXX/XXXX, I have confirmed with the bank that the requirements were met and this was their answer : " After reviewing your account, I see that you completed the requirements, and were met on the statement cycle that ended on XXXX/XXXX/XXXX, please allow up to 90 days for the bonus miles to be credited. '' After more than 90 days noticing that the bonus was not posted to my account I reached again to them and they told me : " After reviewing your account, I can confirm you completed the requirements, please note the bonus miles are processed by the end of the statement cycle of XX/XX/XXXX, please allow up to XXXX weeks for processing, if you have not received the bonus miles by XXXX XXXX you may reach out to us by phone or send another message and we will gladly open a case for further research. '' From that point, the messages kept going back and forth and they were coming up with a different excuse every time to tell me at the end around XX/XX/XXXX that after a thorough investigation, it turned out that I am not eligible for the promotion and I gave up complaining to them at a certain point. 
Subsequently, I am submitting this formal complaint because I judge that Citibank has been very misleading to me in this transaction. If I am not eligible as they claim ; then they should have told me so from the very beginning and I would have understood it. However, they confirmed to me the eligibility and that everything is fine and that I should not worry again and again and again while deducting the XXXX $ monthly fee that the account carries every month. Out of a sudden, their systems show that I am ineligible for the promotion. 
Kind Regards, XXXX.