Try the following steps to troubleshoot the issue:
1. Brush and clean the battery contacts in the battery compartment before replacing them with a new fresh set of AAA batteries. Pay special attention to the battery orientation to avoid unintentional damage. Ideally, please use alkaline batteries as zinc or rechargeable batteries often do not work as well.
2. Make sure you remove the clear plastic insulator piece from the LR44 button batteries that are pre-installed in the battery compartment (next to the UV Light Switch) of the UV stand, prior to first use.
If you are not using the device for an extended period of time, you should remove the batteries to prevent corrosion or battery leakage.