Server side processing hangs

I am trying to get DataTables working at the genetics research institute where I work. After discovering DataTables at RailsCasts, I've followed Ryan Bate's example as far as I can, and tried to study more examples here, to no avail; my application just hangs and says "Processing" forever when I call it. I would really like to get it working though, as it seems the perfect plugin for our needs. However, I am new to Rails, and am at a loss as to how to troubleshoot the problem. (I am going to try and learn how to start using the DataTables debugger, which looks like it could be very helpful.)

Unfortunately, our website is not public, so I cannot provide a URL. However, I suspect the problem is in my /app/controllers/genotypes_controller.rb or my GenotypesDatatable class, which I've provided below; any helpful suggestions will be very much appreciated!

(here the class:)
class GenotypesDatatable
  delegate :params, :h, :link_to, to: :@view

  def initialize(view)
    @view = view
  end

  def as_json(options = {})
    # This is what feeds directly into DataTables
    {
      sEcho: params[:sEcho].to_i,
      iTotalRecords: Genotype.count,
      iTotalDisplayRecords: 10,
      aaData: data
    }
  end

private

  def data
    genotypes.map do |genotype|
      [
        # Note: h is shorthand for html_escape
        h(genotype.gmarker.marker),
        h(genotype.gsample.labid),
        h(genotype.gsample.subjectid),
        h(genotype.gsample.box),
        h(genotype.gsample.well),
        h(genotype.allele1),
        h(genotype.allele2),
        h(genotype.run_date)
      ]
    end
  end

  def genotypes
    @genotypes ||= fetch_genotypes
  end

  def fetch_genotypes
    genotypes = Genotype.order("#{sort_column} #{sort_direction}")
    genotypes = genotypes.page(page).per_page(per_page)
    if params[:sSearch].present?
      genotypes = genotypes.where("labid like :search or category like :search", search: "%#{params[:sSearch]}%")
    end
  end

  def page
    params[:iDisplayStart].to_i/per_page + 1
  end

  def per_page
    params[:iDisplayLength].to_i > 0 ? params[:iDisplayLength].to_i : 10
  end

  def sort_column
    columns = %w[allele1 allele2 run_date]
    columns[params[:iSortCol_0].to_i]
  end

  def sort_direction
    params[:sSortDir_0] == "desc" ? "desc" : "asc"
  end
end



(and here's the controller:)
class GenotypesController < ApplicationController
  # GET /genotypes
  # GET /genotypes.json
  def index
     respond_to do |format|
      format.html
      format.json { render json: GenotypesDatatable.new(view_context) }
    end
  end

  # GET /genotypes/1
  # GET /genotypes/1.json
  def show
    @genotype = Genotype.find(params[:id])

    respond_to do |format|
      format.html # show.html.erb
      format.json { render json: @genotype }
    end
  end

  # GET /genotypes/new
  # GET /genotypes/new.json
  def new
    @genotype = Genotype.new

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @genotype }
    end
  end

  # GET /genotypes/1/edit
  def edit
    @genotype = Genotype.find(params[:id])
  end

  # POST /genotypes
  # POST /genotypes.json
  def create
    @genotype = Genotype.new(params[:genotype])

    respond_to do |format|
      if @genotype.save
        format.html { redirect_to @genotype, notice: 'Genotype was successfully created.' }
        format.json { render json: @genotype, status: :created, location: @genotype }
      else
        format.html { render action: "new" }
        format.json { render json: @genotype.errors, status: :unprocessable_entity }
      end
    end
  end

  # PUT /genotypes/1
  # PUT /genotypes/1.json
  def update
    @genotype = Genotype.find(params[:id])

    respond_to do |format|
      if @genotype.update_attributes(params[:genotype])
        format.html { redirect_to @genotype, notice: 'Genotype was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: "edit" }
        format.json { render json: @genotype.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /genotypes/1
  # DELETE /genotypes/1.json
  def destroy
    @genotype = Genotype.find(params[:id])
    @genotype.destroy

    respond_to do |format|
      format.html { redirect_to genotypes_url }
      format.json { head :no_content }
    end
  end
end