The difference is because Jackmanii is a Type III and Niobe a Type II. Type II's typically get off to a slower start than III's. III's are usually more vigorous in general. Getting lots of flowers the first year isn't typical though of any clematis. They are first year sleep, second year creep and third year leap plants.

I would not fertilize in your zone at this time.

Cut them both back to the last set of buds in early spring when you cut back roses in your area. Then fertilize them.

Thanks buyorsell888. The Jackmanii is flowering like crazy (second flush). When I bought it and planted it in early July, it was flowering (they both were - the Niobe too I mean). I will keep my fingers crossed for the Niobe for next spring. Thanks again.