You said "new post" in your question. Do you mean a new post in an existing thread (a response)? Or are you talking about starting a new thread topic? Since you're not a very frequent poster, you may simply be hitting the wrong button for what you want to do. We've all done it. Heck, I've been posting a few times a day for a couple of years and still do it at times. lol 😉
If what you want to do is reply to an existing thread, you'll want to hit the "post reply" button. If you want to start a new topic of discussion, you'll want to hit the "new thread" button. Since they're right next to one another, it's easy to accidently click on the wrong one. If that happens, just let one of us mods know and we'll fix it for you. If I'm totally off base on what the problem is, you can hit the "Ann's a dumby" button and..... 😛
Ann