Hello Steven!
I am not girl 😊 But I wanted to say my opinion on this matter. I had same thing that some girls did not answer my letters and I was little surprise of that. I thought that I know something of Russian culture. I think it is a good way to respond politely that no thank you, if you are not interested. Anyway now I have found a nice lady and we are going to meet very soon after 6 months communication. I still get letter from womens allmost every day and answer those, I have always answer if someone write to me. I think that is polite to answer.