Karen,

I've heard this complaint before and also have had mothers call about the
plunger sticking.  I don't know if the burping and spraying occurs in
conjuntion with the sticking of the plunger, but would guess it might.  I've
found that if the mothers take the plunger apart and wash and dry it well,
this fixes the sticking.  Why this happens I'm not sure?

Pat Lindsey, IBCLC near Orlando, FL