Oh, assuming you aren't using a very old version of rockbox. I very dimly remember the hold switch didn't work on the 6g at first, but it was fixed a few years ago. Or, rather, I think it worked functionally, but Rockbox didn't know about it so the lock icon would not appear on the WPS.
It's not uncommon for the hold swtich to physically malfunction, especially if it's an old second-hand or refurbished ipod.