Is there something I have missed? I have added the position as per the documentation (http://www.t3-framework.org/documentation/bs3-layout-system#add-position) including updating the templatedetails.xml file.
For reference, this is what I have added, in its context. It is the 'center-block' position which is new.
<div id="t3-content" class="t3-content col-xs-12 col-md-6 col-md-push-3">
<?php if ($this->countModules('center-block')) : ?>
<jdoc:include type="modules" name="<?php $this->_p('center-block') ?>" style="T3Xhtml" />
<?php endif ?>
<?php if($this->hasMessage()) : ?>
<jdoc:include type="message" />
<?php endif ?>
<jdoc:include type="component" />
</div>
The development site page I am working on is here with a dummy module in the new position:
http://www.smsiclinic.co.uk/home-athletematters?tp=1