The fluctuating reading on the D.C. ammeter may be caused by several factors, including unstable power supply, nearby magnetic fields, or issues with the meter itself. Cleaning the contacts of the ammeter is recommended to minimize fluctuations. It's important to clarify the experimental setup, including the specific circuit and current being measured, to diagnose the issue effectively. Testing the circuit without the current balance and observing the fluctuations can help isolate the problem. A systematic troubleshooting approach is essential to identify whether the fluctuations originate from active or passive devices in the setup.