There are a few possible reasons for the issue.

Check the battery performance through the Device Health Check tool to see if the batteries still perform well. Please refer to My TapLinker has been running well. But it didn't water as scheduled in the past few days. What could be the cause? for more details.
If the water timer has been in a drawer for several months, there is a slight possibility that rubber diaphragm in the valve gets stuck in a certain position. To release the rubber diaphragm, you can tap the water timer gently on a hard surface, as shown in this video.
Another possible cause is that the solenoid valve is jammed by scale, debris or objects. What you can do is remove the back case of the water timer, then disassemble the valve and clean it. All you need is a Phillips head screwdriver. This video shows how to assemble the valve.

For further assistance, please email us at support@link-tap.com.