One of my content types will not take a template!

Another way of asking the same question:

How do I completely delete a template so I can start again?

or... How do I check that a content type is "clean" ie. ready to have a template applied to it?

By the way, clearing the cache does not fix this..

I clear the caches in the following way:

<?
echo "This page runs a script that clears the main cache.
";
db_query("DELETE FROM {cache} WHERE 1");
db_query("DELETE FROM {cache_filter} WHERE 1");
db_query("DELETE FROM {cache_menu} WHERE 1");
db_query("DELETE FROM {cache_page} WHERE 1");
echo "It's clear now.
";
?>

So to clarify my question further: is there anything else that I can clear, or delete, or change, so that I can create a template for this one particular content type?

I can create templates for all the other content types, just not this one (I can even create a template for the EXACT DUPLICATE of this one).