Additional information about the type of wall material, flow rate, consolidation time in the hopper, type of feeder/feeder interface, is needed to fully answer the question. From what was given, 1) the wall friction data indicates that a wall angle of 30 (best case) to 25 degrees is required for mass flow. 2) the smaller particle size will be more cohesive and, as a result, more likely to funnel flow or even rat hole with his hopper design (any information on cohesive strength, permeability, density function would help).

I would recommend a material flow property test on the smallest particle feedstock. This would better define the flow patterns, hopper shape and possible limiting flow conditions. Existing test reports may be suitable.

It would be a quick, inexpensive and relatively easy step to polish the hopper walls, depending of course on what they currently are. If they are stainless steel, I would recommend using a scotch pad with a polishing motion along the fall line of flow. If the wall is rough or has rough weld affected regions, use a flapper wheel with 240 grit, again directed along the fall lines of the hopper.

If polishing does not work, I would recommend replacing the cone with a proprietary hopper design which has the advantages of a) mass flow with angles 15 degrees more shallow the conical hoppers, b) arch free flow through an opening half that of a conical hopper, c) twice the limiting flow rate of conical hoppers. We can specify a hopper with additional the additional data indicated.

Details of the hopper/feeder interface are needed to assure that the problem does not begin at this location, as it often does.