Even perfectly sharpened blades can refuse to cut, and there are common culprits:
Detachable Clipper Blade Anatomy
The comb and the cutter each have raised portions at the teeth and heel. The raised portions are where comb and cutter come into contact with one another when the blade set is reassembled. The raised portions are actually bearings and like the bearings in your car, they must stay clean and lubricated.
Do you wash the dogs before or after grooming? Rust is the number one culprit, and can happen in just a few hours. The cause can be due to cutting wet hair. The only cure is prevention.
If your blades are rusty, they must be re-sharpened to remove the rust. If you cut wet hair, cleaning and lubricating during and after each grooming can prevent rust.
Oily residue and varnish can cause sharp blades to not cut. The oily residue comes from grooming an unwashed dog. The natural oils in a dog's coat can collect on the blades. It will build up on the bearings and cause the blades to separate slightly. Then hair clippings will get between the blades. The heat from the blades will build up to turn this mess into varnish. It will continue to build up enough to prevent the blades from cutting. The blade action will slow down. More hair will become trapped, causing perfectly sharp blades to "munch up" and "bog down". The solution to is to clean your blades several times or as much as needed when grooming.
Washing the blades while grooming is very important to the maintenance of your blades, and will keep your blades cool as well as speed up the grooming process. Don't forget to put one drop of oil on both comb and cutter bearings at the end of the day. You do this by pushing the cutter nearly half way to the side and oiling, then doing the same to the other side.
To extend the life of a sharpening you need to clean the blades daily and get the pet dander and Spray type Lube off the blades. If you don't, only a sharpening will get rid of the "crud" because it will become very hard and sticky. Slide the cutter to one side on the blade, is there a red or brown film coating everything? Is there pet hair impacted under the spring? Does the blade feel tight and difficult to slide? If any of these symptoms are present it means you’re not cleaning your equipment properly.
Work Harder! Millions on welfare depend on you.