High humidity can affect engines by raising the "density altitude" and causing an engine to lose a little power and run a bit rich.

Having said that however, other than possibly a very small adjustment to the carb mixture needle(s), it is nothing that would seriously affect the performance of a model airplane engine.

Check and/or replace your spark plug, then check the fuel system. Filter in carb clogged? Fuel old? Regulator diaphragm in carb old/stiff? Etc.

Nobody has ever been able to see or catch one, but sometimes little gremlins go to work on planes between outings. They can cause all sorts of problems....

Good luck,
AV8TOR