Dwarf Hydrangea paniculata

Lovely. What is its height?

Not exactly sure, there are some small varieties that are said to grow 3-4' or 4-6'.

OK. You know, I always wonder why they all have so many flowers when in the shop and when I put them in a place which according to all recommendations should suit them, they stop to bloom.

A lot of it has to do with the attention they're given from the growers who produce them. They are usually potted in media that is much better than any garden soil, fertilized frequently, trimmed back as often as once a week and low quality plants are typically not shipped out. It's easier to control how they grow sometimes when they're in a pot, you always know when to water and can control the environment the plant is in so it always has the right temperature and sunlight.