you have one bad or badly seated or badly valved middle (M) reed
of one reed pair
fortunately, you are in Baltimore where there is a lively accordion scene
if you bring it to the next meeting any of several of us can pop the bellows
pins and diagnose it for you, possibly fix it on the spot
unfortunately, the next meeting is the first Saturday in September
(meets in Catonsville)
if you can't wait that long, bring it down to Wheaton MD. and
have Ike take care of it for you (he is a full service repairman)
it may be as simple as a valve being out of place or perhaps
fell off during shipment and just needs re-positioned/attached
each individual note is a Pair of reeds over a single hole
and keyvalve.. one for the pull, one for the push
they work in partnership and affect each other. the ventilli on one
closes on the push to keep one reed quiet, so the air goes to
the other reed whose ventilli opens. If one has fallen off then the air is
"leaking" through the wrong reed requiring that great push to get enough
air to the right reed to make it sound finally. this is one possible diagnosis