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.

Must be in this format - last name FIRST? Dorm letter?
Also December mail is slow for everyone. But the first one should have arrived.

AB Last Name, First Name, Middle Initial
# # TRS / FLT # # # Dorm “letter” – # (the dorm letter is either A, B, C or D)
1320 Truemper Street Unit 36 # # # #
Lackland AFB, TX 78236 – # # # # (zip extension)

Did you send big cards? Stickers? Smiley faces? Dot i's with little hearts? If this person is a trainee, any tiny little added thing will get them raked over the coals, depending on the officer.
Letters Must be rectangular.
Minimum size is 5″ long x 3 1/2″ high x 0.007” thick.
Maximum size of 11 1/2″ long x 6 1/8″ high x 1/4” thick.
Maximum weight is 3.5 oz.
A letter will be charged a nonmachinable surcharge if it’s a square letter 5″ x 5″ or larger, it doesn’t bend easily, has clasps or similar closure devices, has an address parallel to the shorter dimension of the letter, is lumpy, or the length divided by height is less than 1.3 or more than 2.5.

Exactly how I wrote it.

I've been sending them with just plain, rectangle white envelope. Just plain black text, nothing on them besides the text. The stamps are just normal stamps, such as a stamp of the American flag.

Send one day for a week, just to see what happens.
Can't think of anything else!

Of course US mail did not deliver on 25th, Christmas day,
And with all the holiday mail, it can be delayed. Also mail to military can often take a week longer or so, it goes to the base, where it is then divided and sent to the unit.

There is no directly mail to an individual. It goes to the main base receiving office, is manually sorted, bundled , sent to the company, sorted by platoon(Army), given to platoon leader and then given at mail call. Sending mail is different, the PO picks up and goes from there.