You could check for leakage on the antenna wire with a "megger".  You may also have induced losses in the wall of your house and loss happens to vary with moisture conditions (could be checked before and after a hosing down).  A possible countermeasure is to increase the spacing of the antenna "upwire" from other objects (which reduces the field strength and induction loss in the other surface?).

 

My shack is in the attic. The aerial wire goes up under a slope to the top of a steel mast at 18m. The wire is far removed from roof and walls. There is some unwanted capacitance to the mast of course but it is unlikely that this varies with season. The current flows to earth via the central heating system and the gas mains. As the full aerial current flows via that system it certainly takes part in the radiation. But the voltage on it is so low that no losses via capacitive current to damp walls etc. can play a role.

 

73, Dick, PA0SE