Sweet Autumn clematis (C. terniflora) blooms in August and September and has white flowers, so perhaps that is what you have and it just isn't blooming yet. Can you post a photo of the leaves, and perhaps then it can be IDed in at least a general fashion.

I have a white, Gillian Blades, that is about five years old. It also grew well and looks healthy but has had no blooms. Other years it bloomed in the spring. Hopefully it is just an off year for this clematis which is one of a multitude of clems I grow on my neighbors chain link fence.

My Sweet Autumn generally blooms late September/early October.