Blueberries love three things: 1. wet feet; 2. acid soil; 3. full sun. Are yours getting those? I can't say whether the clay is an issue or not--my soil is well-seasoned, organic-rich sandy loam. The bushes should be fertilized twice yearly--first time on Patriot's Day (on or about April 20), second and last time on Memorial Day. Have you fertilized your plants at all?

My wild guess is the pH is off. Without having any test results, it's hard to tell. However, there isn't anything in that list that can lower pH if that is the problem.

Clay + sand = concrete. I hope you'll reconsider the idea of adding sand, especially since it isn't a reversible step. Add lots of organic matter instead; it helps drainage and will not have the negative effect that sand would.

I'm not just repeating something I've read, this is a problem I've had to deal with in my own garden. Not that I've actually added sand, but our soil is layers of clay and sand in a glacial outwash plain, and if I just mix the local soil in place I get an impervious cement mass.

MadGal and gardenweed have good points. I'd like to add that blueberries a. are best transplanted while young and b. take a while to get established. If you dig them, you may be setting them back more than you'd like.

Last, I've had really good results just adding a thick layer of organic mulch on top of our poor soil. The earthworms get right to work on this, and incorporate it into the soil. They do wonders for the drainage. You might want to apply your compost around (not on) the bases of the plants, top it off with a load of chopped oak leaves, and sit tight until spring.

A few general thoughts:
IME, blueberries are slow to establish. If they are new this year, do you just need to be more patient?
Depending on how well composted your manure is, it may not be acidic enough to make blueberries happy since it initially is somewhat basic. I would suggest that you get a soil test done. Check with your county Cooperative Extension or through UNH to see how best to get that done.
I did work for a blueberry farmer a number of years ago. Her bushes were mulched with either with sawdust or with pine needles. (We collected the pine needles that folks had raked up in their yards and piled on the curb.) It helps keep the moisture levels even.