I have experienced this issue some time too - I think if you post some "controversial" comment (judged by many quick upvotes and downvotes) it triggers a "cooling down" period before you can post a reply to your immediate child comments in the thread (or it could be mod-triggered). This ensures you don't dominate the thread, and allows a conversation with other participants to develop. Based on how others react to the comments, I assume it also gives the mods a better idea if they need to intervene. I found it a minor annoyance at first, but have learnt to appreciate it - thoughtful comments (with careful moderation) from a diverse group of people is what makes a community like this valuable.