Keep writing. That is the most useful thing you can do right now, even without knowing whether she has received anything yet.
There are several reasons an inmate goes quiet that have nothing to do with you or the friendship. Newly incarcerated people in particular often go through a period of withdrawal where they are not ready to communicate with anyone on the outside. The disconnection from normal life hits hard, especially in the early months, and some people need time to process where they are before they can engage with the people they care about. It is not rejection. It is survival instinct.
There is also the practical question of whether your messages are actually reaching her. If you sent a digital message through a platform she does not have access to, or if the facility processes mail slowly, there may be a lag between when you sent something and when she received it. A physical letter sent through InmateAid is the most reliable format because it arrives as something tangible in her hands rather than a notification on a screen she may not have regular access to.
Keep the letters warm and consistent without pressuring her to respond. Write about your life, share small updates, let her know you are thinking about her and that you are not going anywhere. Persistence without pressure is what eventually breaks through the wall that incarceration builds around people.
When she is ready, she will write back. The letters you send in the meantime are building something even if you cannot see it yet.