Why does my hydrangea flower heads only open at the side petals and stay closed in the middle.?

Mine does this, the outer larger bracts look like large flower petals with tiny real flowers in the middle. the middle of the flower head has very few bracts so the small flowers just look like buds but on closer inspection they do open.
can you post a photo to help us answer this? if my expanlation doesnt help you.

From your description I'd say you're growing the "lacecap" variety of hydrangea instead of the "hortensia", which form those round mop heads. This is how they're supposed to look - I actually prefer them to the mopheads myself.

You got there before me Bamboo :-)

Been on here nearly all day cos I've been trapped indoors with a foot injury - bored stiff! Thank god for the internet, and that'll be why I got there before you!

Hope the foot gets better soon

Technically the flowers which do not open are sterile florets!