Just in case you're not aware of this - and apologies for pointing out the almost obvious if you are - but is your rim drilled for presta or schraeder? Sometimes if a presta valve is used on a schraeder rim the hole will be so much larger than the valve that the tube will protrude into the rim and get cut by the hole edge...there are plastic grommets to take care of this or simply used the slice of old tube that someone else already mentioned.