Posted: 02/07/2014 at 22:38
I think 16 inches is a bit mean for a clematis and that 24" or 60cms wide and deep should be a minimum with a mulch of pebbles or expanded clay pellets to retain moisture. A John Innes no 3 would be better for the compost.
However, as I said, they can take a season or two to settle down and get going so be patient but do consider giving it a deeper, wider pot if you can next spring. It flowers best when treated as a group 3 which means you can cut the stems hard back to just above the lowest pair of buds next spring which will make repotting easier.
Plant it 3 or 4 inches deeper than you currently have it as this encourages extra stems to form and thus more flowers. Give it a generous feed of slow release clematis food then and every spring plus weekly top up tonics of tomorite or similar until it starts to flower. Living in a pot makes it entirely dependent on you for food and water.