Colder temperatures can affect the viscosity of the grease or lube on the hammer assembly. Common according to the Weihrauch owners club. As the colder temperatures set in, the hammer is slowed slightly by whatever is lubricating it, thus reducing the power output.
A strip and light greasing of the hammer assembly should resolve this
Brocock Compatto .177 [MTC Viper 4-16x50 SCB]