I don't know specifically about suberba. I have the regularJackmanii and it is vigorous.
Type 3 Clematis like to be cut back to about 12"in the early spring. Probably not too late to do in your area.Give some fertilizer and a drink of diluted epsom salts monthly too. If planted on an angle sometimes can get more stems. If you have done all these things and it is still doesn't meet your expectations, you can always plant another clem right beside it , even in the same large hole.
