There are a number of reasons what you are doing will silently fail, not the least is that you have an antivirus program scanning IMAP traffic and making it very slow. But there can be many causes of these silent failures. Including the hard limits Yahoo places on connections to the server and other connections using phones and tablets that can further degrade the bandwidth made available for Thunderbird to use with the server. Historically it was "sort of" considered that what you are doing should work for a few hundred massages, but as email have become exponentially larger from some sources the numbers IMAP will reliably move like that gets less. In the mean time the network saturation from streaming of video and sound makes timeout failures more and more common. Fundamentally you need to keep the numbers of messages you move from one folder to another down, perhaps set up archiving to move messages daily, or if the messages are on an IMAP server and you are moving them to another IMAP folder on the same server use server side scripting to move the messages rather than the very inefficient process that occurs using local IMAP folders where each message is identified verified with the server then moved to a cached location pending the "move" actually completing.