Ipod nano stop playing when in sleep mode

I've hit the problem where my Ipod Nano stops playing when it goes into sleep mode. I've read all the previous discussions on this and I've huffed and puffed into the head phone jack ... and finally its working (for now) .. not a lot of confidence that it work tomorrow. Really annoyed that before going into sleep mode everything is okay. So why can't the nano recognise that the jack is engaged when it goes into sleep mode , when it can when it first starts playnig.