Make sure you have done a Reset of your iPod recently (since you noticed this problem).
http://support.apple.com/kb/HT1320
(This just restarts the iPod - it does not erase it.)
If that does not help, there should be a Volume Limit setting on the iPod's Settings screen (it may be under Playback). Does it make any difference if you set that to the highest possible setting?
Also, in case there is some corruption in your settings related to volume, at the bottom of the Settings screen, there should be an option to Reset Settings. That will reset all of your settings on the iPod to the default values. See if that makes a difference. It seems like your iPod should resume playing at the same volume level.