1-Blades are dull, it's a fact, blades will become blunt over time. This can be due to several factors and means that cutting edges of the cutter and comb of your blade are no longer sharp. This can be due to corrosion, scratches or wear. Sharpening the blade is required to reface the cutting surfaces restore these sharp edges.
2-The blade requires oil. Where the cutting surfaces of your blade meet and move against each other they are working against friction. This wears the blade, produces heat and will force your clipper to work harder. It can also stop the blade cutting. In order to combat this, blades should always have a little oil present on them. A few drops where the cutter rubs against the comb every 10/15 minutes should suffice. Avoid excess oil as this can contaminate the dogs coat or could run into the clipper causing expensive damage. Blades should always be oiled after use as the can corrode surprisingly quickly.
3-The blade is dirty. Dirt and grease can stop blades cutting and regular cleaning is required. Blade Magic can be used either with a blade wash caddy or very carefully dipping the tip of the running blade into a small tub of the Blade Magic. Care should be taken to dry and oil the blade after cleaning as to liquid should be allowed to enter the clipper.
4-The blade spring is to weak. As well as the cutting surfaces needing have sharp edges, the blade spring needs to be set at the correct tension. Too weak and hair will get caught between the cutter and comb and push them apart stopping them cutting. Too tight and they will heat up quicker, force your clipper to work harder and for both blades and clippers to wear out far quicker.
5-The drive lever of the clipper is worn. When the drive lever wears, this shortens the distance the clipper can move the cutter on the blade, reducing cutting efficiency. The more wear the less efficient the blades will cut. Replace your drive lever when worn or ensure your clipper is regularly serviced.