Traditional Chinese Medicine views lack of thirst as a symptom that may indicate an underlying pathology of dampness or cold. One way to treat this is to take a short round of medicinal herbs in tea form. These herbs will help you to take in more liquids while treating the underlying cause of the imbalance that is causing a lack of thirst.
While you may be experiences a diminished thirst because of underlying symptoms like this, it may also be due to the quality of your water. You may want to try switching the water that you drink to spring water.
Try drinking your water room temperature. Sometimes adding electrolytes or lemon juice can encourage you to drink more water. I also recommend trying to drink out of a straw, as you may find that you drink more water than expected.
All the best on your healing journey!