Amended soil = planted incorrectly, due to how amending affects movement of water into and out of the amended area - from the unamended area around it. Unless the amended area is a big bed. But then there is still the problem of the amended bed reconsolidating etc. after the amendments decompose and filter away. Unless sand or other mineral material is used. But amending with sand has its own problems. Partial branch dieback of shrubs is often due to root rot. Specimen could have been infested at time of purchase or have become infested after planting, especially if amended area around it is receiving and collecting excess water. Believe it or not, cultivar name is spelled 'Diabolo'.

No blooming is NOT because you pruned or did not prune it. The shrub blooms on new growth. I had one in wide open spaces, full sun, exposed to north wind in mid Michigan z4/z5 (depending on the winter we actually got). Not only were the leaves a beautiful black it bloomed like mad beginning about year 4. I suggest you keep it shaped or it will wind up looking weedy in the future. I heavily pruned mine each September into a nice mounded shape. The last fall I lived at that house I cut up to 3 foot off of the growth and it was not a bit sad about it at all. It is huge and a commanding presence in the middle of the back yard. I would say the problem is you are not fertilizing. Get some plant stakes for even feeding and hit it with bloom booster in early May since it blooms in early June in zone 5. This should solve your problems.

I'm in zone 5, Milwaukee area, the biggest one I've seen was about 7 X 6. I wouldn't worry about the size as its pruned easily. Its actually tolerates light shade, just might not be as purple.

Root Rot. I've been growing Ninebarks for years which great success and then this also happened to me - suddenly the plants died. I'm positive it's linked to the mulch I used. I have 2 Diablos flanked by 3 Amber Jubilee's, and then a separate complimentary Diablo opposite of the garden bed. The complimentary Diablo and 1 of the 2 Diablo's are dying... nothing on the Amber Jubilees or the additional Diablo - which I planted at a different time than the other 2. The mulch I used was the Lowes dark brown brand - probably circa 2014 (plants died in 2016). I only used the Lowes mulch initially because I didn't like it much, switching to the Scotts Dark Brown - which I use everywhere now and have no other problems. The only plant and shrubs that exhibit decay are the ones I plant with the Lowes mulch. Around one of the diseased Diablos I also have the mushrooms that are indiciative of Armillaria Root Rot - 1st time ever.