Kris..did you hard prune your "Henryi" this past Spring?..hard pruning a pruning group 2 can give smaller blooms on the first round of flush but larger on the second..try giving your clems some rose or Tomato fertilizer instead..you want the middle number of the N-P-K ratio ie the "P" to be higher than the other two for bloom power..
yes, you can grow any clem in pots...at least 18" X 18"..and they must drain well..consider your zone as to protection in winter as well..I know nothing about JB's ..don't have that problem here..Jeanne

No I didnt hard prune, just pruned to about a foot thats all.

Can someone tell me if I should "Deadhead" Clems like people do for roses... to rebloom?

If so how do I deadhead? Or shall I leave an old bloom to wither away by itself?

Pruning down to 12" is considered hard pruning..."Henryi" is a pruning group 2 which means he blooms on vines that grew from prior growing seasons and hard pruning will delay bloom period and can cause smaller blooms....Yes, deadheading all plants including clematis is advised if you are not wanting mature seeds for planting..removing seedheads is important because your plant uses up alot of energy making seedheads when it could be using that energy to grow more vines and produce more blooms...always fertilize after the bloom cycle is over and water well...Jeanne