Clematis love very rich soil with good drainage. You are supposed to have almost 2 foot of compost/soil underneath it and they badly need airflow threw it. Don't be afraid to prune accordingly. Obviously you can't lift it now but maybe put some manure on top of the roots and use seaweed feed. Hope this helps.
@edelweiss001. Thanks so much for the advice! Actually, I might need to purchase a new clematis as the rabbits severed the main stem since this picture was taken😥. If I do decide to purchase another one, I will certainly dig a deeper hole for it.