On Thu, 01 Nov 2012 13:33:26 -0500, The Daring Dufas
wrote:

Most tire valve 4-way tools have a thread chaser and reamer that could
fix the valve stem so the core will seal. It's possible that it's a bur
or piece of trash stuck in the threads of the valve stem. ^_^

If the core is not bent or cross threaded.