Your clematis are not too old to bloom well. They just need rich, moist  soil with their tops in the sun and their roots in the shade. Perhaps nearby trees have grown to provide too much shade? I suggest you add Osmocote Plus fertilizer now, water well then top with a two inch mulch of wood chips. Limb up any trees so the tops of the clematis get more sun. Then wait. You should see a flush of new growth and flower buds soon. If not, write back and we will try something else. Gardening is an adventure. Keep growing, Marianne Binetti