I'm no expert, but I was having the same problems and I think mine were resolved by two things:

1. It took a while for the acidifier I used to break down and soak into the rest of the soil before I got a uniform ph drop, and

2. I stopped watering with the hose water, and use only rain water.

If you can't save and use rain water, then you should definitely add something to lower the ph. I don't have to worry about it, so I cant provide much more detail than that. If you do a search on this forum with the term, "bicarbonate" you will find plenty of information and a few heated debates

Hi Joe,
Are these in containers or in the ground?I'm guessing it's the pH of the water that's causing the unwanted reading.Probably the next step is to measure it.
Personally,I only use Pine/Fir bark mulch and Peat moss at about 60/40 whether in a pot or ground.
The Sulfur could take up to a year to work.I also use Sulfuric acid(battery acid)to reduce pH. Brady

Two very good responses. It does not matter what soil you use if your water is high in bicarbonates it will dictate your soils PH.

Sulfur does not directly lower soil pH. It has to be metabolized by bacteria that live in the soil. There are bacteria that convert sulfur to hydrogen sulfide, and there are other bacteria that consume hydrogen sulfide, and convert that to sulfate ion, which lowers soil pH. Bacteria need some warmth and moisture in order to metabolize sulfur. Here in Wisconsin, they are active in the spring, summer, and fall, but not so much in the winter, when the ground is frozen. It can take two years for an application of sulfur to be completely used up. If the leaves are green on your blueberry shrub, that is a sign of acidic soil, and a healthy shrub. If the leaves begin to turn yellow, high pH is the likely cause. How are you measuring pH?

Another problem is it is near impossible to keep bacteria alive in a pot. So you need to make sure water is acidic, and fertilizer is too. The other responses are right on. Peat and pine bark are best. The PH of that mix is about 5.0. At least what Al says who invented the 5-1-1 mix. I do add more peat and compost too in the spring, but mine are in raised beds. So the sulfur also works for me. i prepared beds a year ahead of time.