There's a wearing interal part called the blade drive assembly. It's the little plastic bit that pushes the blade cutter from side to side. They wear out and need to replaced on a fairly routine basis.

This is it on Amazon.

The symptoms are diminished cutting ability like you described. Sometimes you can tell it's broken just by wiggling the external black bit. If it wiggles a lot, then it's broken.

Unplug the clippers, take off the blade latch and the back of the clippers and examine the existing blade drive assembly. You might see that the little white fin bits are broken off at the base where it attaches to the clippers.

If so, just swap in a new one and you're good to go. All you need is a phillips head screwdriver. Easy peasy. Just be sure to unplug the clippers before opening them up or you'll electrocute yourself.

One of my staff literally kept buying new clippers at $200 a pop every time this thing broke.