GE won't agitate

My GE washer will not agitate. It does all other functions (fills, drains, spins out, etc) but after it initially fills the agitator just sits there and does nothing. There is a humming noise but no movement. Any ideas? The machine is 4 years old. Thanks

I pulled the agitator and indeed the plastic hub was worn out. Replaced it and it works as good as new. Thanks for your help.