Belle of Woking is a type 2 clematis and blooms will form off of old wood, I'm wondering if perhaps you cut it back this spring. If not then it could just be the cold and wet spring we've had so far. I know that my Proteus is a little slow with the lack of sunshine we've had. I looked Belle of Woking up on the COTW and it says that she should bloom May, June and a second blooming in September. I'm sure you will see some buds and blooms this summer, may just have to wait a bit longer is all.

Tracy

Here is a link that might be useful: COTW

And it is still early in your zone 4

I don't have B of W, but I do live in a similarly cold zone. One of the issues with my type 2's (I have Niobe, Guernsey Cream, Daniel Deronda, The President, and HF Young as my only type 2's) is that often winter prunes them to ground level so they don't get the really early flush of blooms that many folks in warmer climates get. This year, Daniel D and G. Cream are the only ones that retained old wood, so they will bloom in the next few weeks, while the others may not bloom until summer. I have not yet gotten a second flush of blooms on any of them - I think my growing season is just too short. Depending on how fast your soil warms up and how short your growing season is, you may have some of the same issues with the type 2 pruners that I have. I mostly grow type 1 (Attragene group, which are just starting to bloom now) and type 3, which do best pruned hard each winter.

I forgot to add that my Attragenes don't seem to have the winter die back issues that the type 2 large-flowered hybrids have.