Why isn't my letter being sent to a military base (US)?

I’ve been sending letters to someone from NY to a military base in TX. Specifically Lackland. According to the person I’ve been sending letters to, they have not received any although I sent 2 letters, December 8th and December 18th. I was informed by the person that they have not received ANY letters yesterday, December 25th. The address is correct, it's the same address on the back of the envelope sent to me. Not only that, but even if it was a little off, the mail should be sent there anyway. The full address does contain the squadron number, the flight, etc. Letters have been sent to other addresses fine. I do not have certified mail. The person tells me that their flight gets mail calls and others have gotten mail and none from me. Not only that, but I’ve received many letters from the person already. The recent one was sent the 22nd and I got it this morning.