Select the iPod shuffle in the iTunes sidebar, under DEVICES. Over to the right, on the Summary tab, if there checkbox there for Limiit maximum volume, make sure it is NOT checked. Or, if you want it checked, make sure the slider bar is set to a high-enough volume.
If that is not the problem, maybe the volume control button on the headphones cord is becoming faulty. Are you able to make the volume go down AND up (within a range that is lower than desired)?
You can also try doing a Restore on the shuffle in iTunes, which will erase the shuffle and set it to default ("factory") settings.