The red box frames the encoder identified on the pcb as enc, this sensor records the scrolling movements of the wheel. The red Arrow points to the scroll wheel. Inspects this two components, search for any sign of loose connections, dust, or particles in the encoder, signs of corrosion.

The rest of the arrows point to: green -> middle click button; blue -> left click button; black -> right click button.

Now that the encoder is accessible from any direction, use a dust blower, any compressed air and a brush to blow eventual dust trapped in the sensor from different angles.