I have this report that in http://localhost:8069/report/html/chc.report_loading_dock_vehicle/3 (the html version) works fine, but printing the pdf report or by going to http://localhost:8069/report/pdf/chc.report_loading_dock_vehicle/3 is missing the table of assigned_po. My version of odoo is 10 and wkhtmltopdf is 0.12.1. It shouldn't be a problem with wkhtmltopdf because the sale order report for example works and it has a table of order lines.

Here is the html template:

<odoo>
  <data>
    <template id="report_loading_dock_vehicle_document">
      <t t-call="report.external_layout">
        <div class="page">
          <h2>Roadmap: <t t-esc="time.strftime('%d/%m/%Y')"/></h2>
          <p><span t-field="doc.name"/></p>
          <div class="row">
            <div class="col-xs-2">
              <label>Departure N°</label>
            </div>
            <span t-field="doc.id"/>
          </div>
          <div class="row">
            <div class="col-xs-2">
              <label>Roadmap Date</label>
            </div>
            <span t-field="doc.load_date"/>
          </div>
          <div class="row">
            <div class="col-xs-2">
              <label>Transport</label>
            </div>
            <span t-field="doc.vehicle.id"/> <span t-field="doc.vehicle.name"/>
          </div>
          <div class="row">
            <div class="col-xs-2">
              <label>Driver</label>
            </div>
            <span t-field="doc.vehicle.driver_id.id"/>
            <span t-field="doc.vehicle.driver_id.name"/>
          </div>
        </div>
        <table class="table table-condensed">
          <thead>
            <tr>
              <th rowspan="2">Business Name</th>
              <th rowspan="2">Delivery Address</th>
              <th rowspan="2">Commune</th>
              <th rowspan="2">Guide N°</th>
              <th rowspan="2">Prod. Qty.</th>
              <th rowspan="2">Kgs.</th>
              <th rowspan="2">Volume</th>
              <th rowspan="2">PO N°</th>
              <th rowspan="2">SO N°</th>
              <th colspan="2">Delivery Time Construction</th>
            </tr>
            <tr>
              <th>Arrival</th>
              <th>Departure</th>
            </tr>
          </thead>
          <tbody>
            <t t-foreach="doc.assigned_po_ids" t-as="po">
              <tr>
                <td>
                  <span t-field="po.construction_contact"/>
                </td>
                <td>
                  <span t-field="po.street"/>
                  <span t-field="po.street_number"/><t t-if="po.street2">,
                    <span t-field="po.street2"/>
                  </t>
                </td>
                <td>
                  <span t-field="po.commune_id.name"/>
                </td>
                <td>
                  <t t-esc="po.waybill_id.id"/>
                </td>
                <td>
                  <t t-esc="len(po.preparation_line_ids)"/>
                </td>
                <td>
                  <span t-field="po.total_weight"/>
                </td>
                <td>
                  <span t-field="po.total_volume"/>
                </td>
                <td>
                  <span t-field="po.id"/>
                </td>
                <td>
                  <span t-field="po.source_order.id"/>
                </td>
                <td></td>
                <td></td>
              </tr>
            </t>
          </tbody>
        </table>
      </t>
    </template>
  </data>
</odoo>

And here is the report action definition

<odoo>
  <data>
    <template id="report_loading_dock_vehicle">
      <t t-call="report.html_container">
        <t t-foreach="docs" t-as="doc">
          <t
            t-call="chc.report_loading_dock_vehicle_document"
            t-lang="res_company.partner_id.lang"/>
        </t>
      </t>
    </template>
    <report
      model="chc.loading.dock.vehicle"
      id="action_report_loading_dock_vehicle"
      name="chc.report_loading_dock_vehicle"
      file="chc.report_loading_dock_vehicle"
      report_type="qweb-pdf"
      string="Roadmap"/>
  </data>
</odoo>