Can you make us a few pictures or even better pack your property tab builder (ptb) template and the part/assembly template in a zip file.
Without more description or one of the above we can only guess the case and possible solutions for your problem .
Can't say exactly what cured it. I did reboot the machine during my attempts, which initially had no effect. I shut down my computer daily, Now after 3 days I can access my custom properties template in assemblies. In fact I can see each version I created using the "create now" button. I am using SolidWorks 2015 and point my file locations,templates... to a location on our server.