2 .. After a period of time, the crystal oscillator ages, gradually reducing the output and producing a small fog.
3. There is something wrong with the circuit, such as the capacity reduction or leakage of the positive and negative expensive capacitors.
4. The parameters of electronic components change, resulting in incorrect voltage, such as variable resistance, which reduces the oscillation amplitude and causes small fog.
5. If it is a brand-new component, besides its own quality problem, it may be that the power supply voltage is too low. Try to increase the power supply voltage and see the capacitive withstand voltage of the circuit.
Although the circuit is very simple, there are only a few components, but if you are not familiar with the circuit, then I suggest you buy a new, inexpensive one, and try it before you buy it.