What’s killing my dahlias?

I have four dahlias in my border, but recently the numerous buds have started rotting before flowering - see pictures. The buds form, but never open.
One plant had lots of black aphid looking bugs (?), but it’s occurring on plants with no bugs present.
I feed with feed suitable for dahlias.
Last year, a different variety of dahlia flourished in this spot.

No idea why they’re dying. Any advice is appreciated.

 

I think that the one you are holding is possibly a bud that has finished flowering,the other two in the picture are buds yet to open.

 

Thanks Pete, but I tend it and dead head regularly (every day almost), so I know this one didn’t flower. There are many that bud and go mouldy, or so it seems.

 

Well cant really think what is going on then
Has it been really wet where you are?

 

The only thing, in that case, is a pest P4ul_17. Earwigs are notorious dahlia pests but
I wonder if a caterpillar is present. Have you had a look for pests?

 

The leaves look healthy so it could be something going on in the roots. Too wet perhaps? Have they been waterlogged recently? If you have a little dig around, are there any pests such as Vine Weevil grubs around?

The dark foliage looks great, what kind of Dahlia is it?

 

I've looked at my dahlias today and I'm having second thoughts.
I've only got three, and they are a different type to those in the picture, but I'm just wondering if those pointed buds are seed heads as well?

The new unopened buds on mine are a totally different shape.

Dead flower head.

New flower bud.

 

Hi all,
Thank you all for the responses and @pete , thank you for the files. I took samples to the garden centre and left embarrassingly convinced that they were indeed old flowers. I'll put it down to a learning experience. I didn't realise that the flowers and buds came and went so quickly.

@Selleri , they are a type called Happy Single. First time we've grown them this year and they're brilliant. So many flowers... once you deadhead correctly.