Dropdown list ซ้อนกันทำไงได้บ้างครับDropdown list ซ้อนกันทำไงได้บ้างครับ ใช้ z-index แล้วก็ไม่ขึ้นครับ

                                <td class="xWPosAdsPosition">
                                    <div class="input-group">
                                        <input type="hidden" id="ohdPosAdsPositionSelect" name="ohdPosAdsPositionSelect" value="<?php echo $aValue['rtPsdPosition'];?>">
                                        <select class="selectpicker form-control xWPosAdsPosition" data-size="5" id="ocmPosition<?=$nKey;?>" name="ocmPosition<?=$nKey;?>" maxlength="1" disabled>
                                            <option value = ''><?php echo language('pos/posads/posads','N/A'); ?></option>
                                            <option value = 'TL'<?php echo ($aValue['rtPsdPosition'] == 'TL')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionTL');?></option>
                                            <option value = 'TM'<?php echo ($aValue['rtPsdPosition'] == 'TM')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionTM');?></option>
                                            <option value = 'TR'<?php echo ($aValue['rtPsdPosition'] == 'TR')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionTR');?></option>
                                            <option value = 'ML'<?php echo ($aValue['rtPsdPosition'] == 'ML')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionML');?></option>
                                            <option value = 'MM'<?php echo ($aValue['rtPsdPosition'] == 'MM')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionMM');?></option>
                                            <option value = 'MR'<?php echo ($aValue['rtPsdPosition'] == 'MR')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionMR');?></option>
                                            <option value = 'BL'<?php echo ($aValue['rtPsdPosition'] == 'BL')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionBL');?></option>
                                            <option value = 'BM'<?php echo ($aValue['rtPsdPosition'] == 'BM')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionBM');?></option>
                                            <option value = 'BR'<?php echo ($aValue['rtPsdPosition'] == 'BR')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionBR');?></option>
                                            <option value = 'AL'<?php echo ($aValue['rtPsdPosition'] == 'AL')? 'selected':''?>><?php echo language('pos/posads/posads','tAdsPositionAL');?></option>
                                        </select>
                                        <span class="input-group-btn">
                                            <button type="button" class="btn xCNBtnBrowseAddOn xWBtnShowPic">
                                                <img src="<?php echo base_url().'/application/modules/common/assets/images/icons/View20.png'?>">
                                            </button>
                                        </span> 
                                    </div>
                                </td>