When does your clematis normally bloom? If it's a spring bloomer and it has suffered winter injury or has been cut back severely, it may not be blooming because the flower buds were killed over winter or removed by pruning.
Another possibility: Has the area gotten shadier from growth of surrounding trees? Clematis will not bloom well in shade. It may need to be moved to a sunnier location, or get more light into the area by thinning the tree canopy.
Yet another possibility is that the clematis is receiving excess nitrogen fertilizer. Even if you're not fertilizing it directly, nitrogen fertilizer from a nearby lawn could be affecting the clematis. Cut back on the amount of fertilizer high in nitrogen. Instead use only fertilizer where the phosphorus and potassium content (second and third numbers on the fertilizer label) are as high as the nitrogen content (first number on the label) in the area.