You don't need to 'feel worked' for your muscles to be appropriately and adequately worked. What the CNS 'feels', what condition your nerves are in does not directly or necessarily correlate to stimulus that your muscles need to grow.
When training using HST protocol; regular frequency, progressive and sufficient load, minimum of volume, you are going to 'feel' under worked, but your muscles will be getting the stimulus they need. If you increase the volume unnecessarily, there's a good chance you will 'feel' more worked but will also be approaching an overload of the CNS which ultimately means over-training and;
a) you'll have to SD sooner to recover from CNS overload
b) you won't be making gains because over-training will prevent you from continuing to increase the load. Without the load progressively increasing, you won't continue to grow. Load is always more important than volume.
When training using HST protocol; regular frequency, progressive and sufficient load, minimum of volume, you are going to 'feel' under worked, but your muscles will be getting the stimulus they need. If you increase the volume unnecessarily, there's a good chance you will 'feel' more worked but will also be approaching an overload of the CNS which ultimately means over-training and;
a) you'll have to SD sooner to recover from CNS overload
b) you won't be making gains because over-training will prevent you from continuing to increase the load. Without the load progressively increasing, you won't continue to grow. Load is always more important than volume.