Sometimes when you click into sensor data, the chart may appear empty or that the data is missing. This is because what you see is very dependant on the date filter that you apply and how long it's been since the last time the sensor had a reading.

If there hasn't been a single data point within the filtered time selection that you set then you'll not see anything on the chart, even if there is a datapoint in the older data.

For example, if you have leak detection sensors installed and the sensor data has been in the same state of "no water detected" for 2 months, this is what it will look like even though the device is online.

And when you look at the last time the sensor had a reading that was different and showed "water detected" it will coincide with what you see from the Sensor list view.

So the sensor still works and is still collecting data, it just may not look like it if there has been a long gap between different readings.