The silence is painful, and it almost certainly has nothing to do with you.
The early stretch of incarceration is one of the most emotionally turbulent periods a person can go through. The wave of emotions that hits when someone first gets locked up does not follow a predictable pattern. It moves through sadness, anger, shame, denial, and back again, sometimes all in the same day. Some people shut down entirely during that stretch because engaging with the people they love on the outside makes the reality of where they are feel more unbearable, not less. A letter from someone who cares is also a reminder of everything they have lost access to, and that can be harder to sit with than the silence.
There is also the adjustment period to the physical environment itself. New routines, new social dynamics, figuring out where they fit in a system that has its own rules and hierarchies that have nothing to do with the outside world. That adjustment consumes more energy than most people expect, and writing a letter requires an emotional availability that may simply not be there yet.
None of that means they do not care or that the relationship is over. It means they are going through something hard and have not found their footing yet.
Keep sending. Keep writing. Keep showing up through the mail even when nothing comes back. The consistency of your presence during the period when they are at their lowest is something that registers even when they cannot respond to it. It builds something. When the wave passes and they come up for air, the stack of letters and pictures from someone who did not give up is going to mean more than you can know right now.
Give it a little more time. It gets better.