Is it possible that the speaker wire which connects to the midrange driver is either unplugged or failing to conduct? Unless you're biwiring your mid and tweeter, the feed to the mid is what feeds signal to the tweeter via a jumper so my guess is that signal is not getting to the mid. Check the speaker wire connections at both ends and, if no improvement, swap the left and right speaker wires to see if the problem persists or migrates to the other channel. Otherwise, the only thing I cam think of is if there is no output from the corresponding channel of your amp which drives that speaker or if the preamp or the source component(s) upstream have dropped a channel. If the problem persists with more than one source, I think you can rule out sources. But the preamp, or any of the ICs that run from the source to preamp or preamp to amp need to be ruled out by swap or substitution.
