Could be badly meshed gears. If you cut some copy paper into thin strips bed this into the mating gear teeth as you assemble them onto the banjo which will give about the right clearance. Just work the gears back & forth buy hand and feed in the strip, it will look like a concetina.
The other thing that can cause problems are the washers that space the gears off the banjo, they are slightly different thicknesses and if mixed up you can get the sides of the gears rubbing each other.