Those who get low readings are either out of ketosis or using them as fuel (I get this question a lot).
Being in a state of ketosis, means that you have achieved a blood ketone reading of 0.5 mmol/L or higher. This means that there is an abnormal amount of ketone bodies circulating in your blood, that is causing a reading of higher than 0.1 mmol/L (having a blood reading of 0.1mmol/L is considered normal for a healthy individual following a typical diet). If you are following a ketogenic diet, or are coming off of a prolonged fast, this elevated level of ketones in the blood is a good thing, it means you are on your way to using ketones as fuel. However, just because blood ketone levels are elevated, does not mean they are being used as fuel just yet.
Using ketones as fuel, should decrease the amount of circulating blood ketones because ketones are being pulled out of the blood, and into the cell to be used as fuel. If you have fully transitioned to using ketones as fuel, it is completely normal to experience a drop in blood ketones, even if you have been following a ketogenic diet. Once you have fully transitioned to using ketones for fuel, your blood ketone levels may rise again, although this may change from person to person.