Joshua,

If the velocity in the main duct is extremely high, it could be sucking
air into the duct. The high velocity pressure of the air in the main
duct can lower the static pressure in the duct below the room static
pressure. This aspirating effect is used in chemistry labs to produce a
vacuum in a system by hooking a hose as a side tap to another hose with
water running through it. It can also be demonstrated with a simple
experiment. Hold the short end of a dollar bill directly below your
lower lip, with the other end of the dollar bill hanging down. Now blow.
The dollar bill will fly up like a flag. Your high-velocity air is like
the air in the duct, and the low pressure in the main air stream is
sucking up the dollar bill like the air in the duct is sucking air out
of the room. I recommend estimating the velocity pressure and the static
pressure at the problem points to see if aspiration is causing the
problem.

Sincerely,

Keith Swartz, PE, LEED(r) AP