My dehumidifier doesn't seem to be working, but my indoor humidity is around 22%. Is this normal?
Take my advice with a grain of salt – I live on an extremely humid island – but my dehumidifier can be set not to work below certain relative humidities, and those are 40%, 60%, and 80%. A quick googling turns up a number of sites that suggest 40%-50% as the ideal range for health and comfort. 

It may depend on where you live (Arizona?), but the 16%-22% you're seeing strikes me as very low. If that's what humidity is like throughout your house, even without the dehumidifier, I think it's safe to say you don't need one. I would wager that they don't really make consumer dehumidifiers that can remove moisture too far below that.