hi all , please dont think im daft or strange , but i really think my 2 year old moggie is sad

ive noticed for about the last 6 months she never purrs , by never i mean never , she looks ok in her self, she still comes for playtime etc shes eating really well , not being bullied by any of the neighbours cats , she gets on with my 2 dogs or (are older than her)
my question i suppose is , is it ok for some cats not to purr? or is she a sad cat
btw i have the feriway (sp) plug in thingy for happy hormones !! thats for stella not me !
