The nipple should move freely in the collection tube. If the nipple rubs painfully against the sides of the tube, that indicates a too-small shield. If there is a loss of suction, the shield is probably too large. Either problem can result in lowered pump effectiveness.
some people just don't respond well to the pump; there are lots of things you can do to troubleshoot but on occassion nothing rally works. hand expression often works better for those people. and you will want to keep the milk flowing while you're away so you have supply left when you return, one way or another.