Mousewheel not working properly

I purchased DELL KM3322W Keyboard 
& Mouse Combo 

With Warranty: 3 Years Domestic Warranty

Since I purchased this product 5 months back so I am in the warranty coverage.

Problem: Mouse wheel is not working correctly now. It's kind of sliping or something. So when I scroll with mousewheel it doesn't scroll the page like it used to when it was new.