This could be for several reasons. Some questions which ask or reveal personal information won’t be answered publicly for privacy reasons. If you contacted me anonymously (and the only way I can answer is publicly), it leaves me a little stuck – it’s always best to give me a way to get back to you. I may also choose to answer questions a little later than when they were asked so platforms don’t get overloaded with Q&As. I can’t answer all the messages I’m sent publicly because there’s too many of them.
If you’ve been in touch privately, it could be that I haven’t had the time to read or respond to your message. I try to respond as quickly as possible. If you’re getting impatient, pester.