they won't exchange !

My husband purchased a baby bouncer for my 6 days baby....when I opened it I found out that it was too small I wanted a bigger one...I went there to exchange and I was told that they couldn't exchange apparently there is a stain that they can only see....it's still in plastics I have never used it.....my baby is too small to even use it for now.....I tried to explain to the manager but instead she made me look stupid.....