Although replying to a message is easier than starting a new message, it creates additional work for the receiver since they must sort out your issue from the issues originally being discussed in a thread.

Another form of this problem occurs when someone introduces a new but similar problem in the middle of the thread for working on a problem.

In both cases the best outcome is that the receivers are confused by the two issues, at worst your problem gets completely lost. If you’re introducing a new subject, start a new thread.