Why have my dahlias stopped growing?!

[SconNotScone]

I planted 8 dahlia tubers in the middle of May, directly into the ground. This is my first time growing dahlias, I followed the instructions around depth and spacing to a T. They started to grow, all was good I thought. Some varieties have grown taller and spread more than others, but over the past 3 weeks or so, I think they haven’t grown even a centimetre! Two of them are very small - probably only 15 or 20cm tall at the moment. I am seeing other people with their dahlias starting to bloom already, and i’m worried about why mine have completely stalled.
I am keeping them watered well during the hot weather, they get plenty of sunshine and are well sheltered. What might be going on?! I was so looking forward to having lots of my favourite flowers to enjoy over the late summer!!

Hmm - not a gardening expert, but will say mine that have been grown from tubers this year and have big buds on. And the established ones are almost blooming. All semi-shaded.

Maybe it's been too hot? My huge hydrangea bed which looked so promising mid-June (I have been away and only just back, although the gardener has been here most days watering) now look a bit tragic.

Were they all the same variety or a mixture? I have got dwarf dahlias from seed just beginning to flower. Last year they grew ok until the slugs had their wicked way. They are about 6'' tall.

A mixture of varieties, but none dwarf, they are all supposed to go to between 70cm and 110cm, if I remember correctly.
There was a little bit of slug damage to the lower leaves, but actually not too bad.
I am so upset!! I have been for a closer look, and can actually see two buds in one of the plants, but nothing whatsoever on any of the others.