Since we don't know what kind of clematis you are growing, it's impossible to know if any are those which have a later bloom season - not all clems bloom in spring :-) But they do sound pretty young to be putting on a show like an established vine would. I allow at least 3-5 years before I expect my clems to put on a good show. IME, mulching well with compost and an annual topdressing with a rose or tomato fertilizer is all that is necessary. No 'bloom boosters'.

A light shearing or trimming back of your spring bloomers could promote a later flush of blooms but not all spring blooming clems are repeat bloomers. Sometimes patience is your best garden ally :-))

Thank you, Gardena. My new ones, planted last fall are Josephine and Piilu. Josephine has not bloomed and Piilu did, probably a dozen blooms. I planted Huldine this spring. Lots of full foliage but no buds yet. I have a purple one, name unknown, that is about three years old and bloomed amazingly! I also planted sweet autumn last fall and hope it does something for me this year. They all look pretty healthy and I'm excited to see what they become!