If these were one or two gallon size plants, chances are they were simply too immature to set buds last summer/fall. Even for larger sizes, it's quite common to not have any bud set in the first year after planting out.

Given your experience with the root balls, it is probably worthwhile to dig up one or two to see what shape they are in even if this has little or nothing to do with the lack of bud set.

Seriously matted root balls need more than a bit of prying with a shovel. You need to eliminate circling roots at the edges and bottom. This may mean cutting with a sharp knife, using a hand cultivator as a claw and/or a forceful stream of water from a hose to expose1-2 inches of the root ends. You'll lose some roots and think you're committing rhododendron murder, but it's the only long term cure. Once that's done, water by immersing in a bucket for the ones you dug and letting a hose trickle at the base of the plant for a half hour or so each time they need water for the rest and after replanting. A sprinkler hose will not wet a dry peat-based root ball. You want to wet the root ball, not the surrounding soil. Saturated soil makes it more difficult for the roots to extend out from the existing ones. You may have to water like this for a second year as well.

Yes, more sun in June and July may stimulate more flower buds for the following spring. Since you bought yours in July, it may have been the shade at Home Depot combined with the current shade that is causing the lack of bloom. You may be OK next year.

I agree 100% with opening up the roots, soaking for an hour and replanting.

Here is a link that might be useful: How to plant (Rarefindnursery).

Dear Rhodyman and Maingrower,
Thank you for informing me that the rhododendrons simply may not bloom the first year out. I did dig up one and set it in the water for a half hour after doing the major cut as suggested. Yes, I did feel like I was committing rhodie murder, so it was helpful that you had told me I might feel that way. I did replant, we shall see if one more year works wonders.
Now, I see my rhododendron maxima in a different part of yard have no blooms set either. Sigh. I see this rhododendron mania requires patience.