It is aluminum that gives Hydrangea macrophylla the blue flowers. If the soil is not sufficiently acidic, the aluminum is bound up and not available for easy plant access. It sounds like you are trying to maintain proper acidity but if the AL is not present in sufficient levels, even acidic soil will not produce the proper coloration. If you can get it, look for aluminum sulfate, sometimes known in this country as "hydrangea bluing" and apply according to label directions. Do not overapply. It takes awhile for it to become effective so you very likely will not see any results this season but you should next year.

gardengal48

Thank you very much for your nice advice. I went to RONA store and I bought 1.7 kg aluminum sulphate. On the package it says: this will change Hydrangeas to blue. I was sooooo happy when I bought it. I started to add it to my plants. I hope next year to see blue Hydrangeas ? Thank you again