metra engine wheel slip

when i first purchased my kato metra set everything was fine. now a month or so later when i climb about 2% grade the wheels start slipping. i cleaned the track and wheels. still nothing. any tricks to prevent this wheel slippage. thanks for any help.

Did the loco originally have traction tires and they have come off?

That would be my first thought.

Good luck,

Richard

no oddly enough no traction tires.