Fiirst, no clematis blooms all summer. Pruning group 2 clems (also known as the large flowered hybrids) typically start blooming in May (sometimes as early as April) but usually fizzle out by July. With a light trimming at that time, many will produce a second bloom flush in late summer or early fall.

Pruning group 3 clems - like Jackmanii - can start blooming as early as mid June (July is more typical) and will often continue on until well into fall. But these flower on the current season's growth and so look their best if cut back hard every year in late winter. Jackmanii is one of the most prolific blooming and easier clematis to grow. Try pruning it more severely (down to the second or third set of buds from the base or about 12-18") in March and fertilize after pruning with a good rose fertilizer. Mulching well with compost won't hurt either. I'd bet you see much better performance from this vine going forward.

I'm in zone 5 and my longest blooming clem is Rooguchi, hands down, but it has a smaller bloom than you may want, and it's bell-shaped, but I think it still makes quite an impact, and it's very easy for me.

Yes agree with gardengal, Jackmanii a winner for show and hardiness your zone. Is he new? All clems can be erratic first few years and then, wam! Gorgeous!