If the temperature is fluctuating, check the water flow rate.
This may be caused by inconsistent water flow/pressure or others using water outlets/taps in the house.
If this is fluctuating, then this causes the temperature to do so too. This leads to a lower alcohol % in the final output.
If using the Turbo 500, adjust water flow by opening/closing the water flow controller valve. Ese the Still Spirits Water Flow Regulator if you have one. If your temperature is still fluctuating with the use of the Water Flow Regulator, please view this article: Why is the temperature still fluctuating even with a Water Flow Regulator?
It may also be caused by frothing of the wash up the column in which case, make sure you are using distilling conditioner and ceramic boil enhancers. This can also be caused by unfermented sugar in your wash.