Walk Behind Lawn Mowers

I have a 2 year old HU700f and have had problem with the front wheel drive. After the first year and only four uses the front wheel froze up. I took it back to the Lowes I bought it from they in turn sent it off to the repair site and they charge me half of the repair cost of $160 because I didn't buy the extended warranty. Now the inside of the front wheels where the teeth of the steel gears meet the plastic wheel has been chewed out. Just wondering is this normal?