no response/acknowledgement?

On PC using what browser? I have no issues with it, it's how I keep track of everything I have read. I did notice though, that if you mark all threads read using your phone, then come onto your computer and there are new posts in the thread in the meantime then it will drop you to where you last read on that specific device. To simplify it, I mark everything as read on my phone, at that time this thread has 3 posts and I had last checked it when it had 2 posts on my computer. If I immediately come on the thread won't show up, because it is read. But if someone puts a 4th post in here rather than starting at the 4th it will start at the 3rd for my computer.

Cleared your cache? I'm using firefox, no issues, so it's most likely a local issue.