I think we would need a bit more detail. The plot sounds way too small for two pyracanthas, but that wouldn't explain the sudden dying. Is the soil dry or moist? Shaded or full sun? What time of year did you plant them? How long were they growing before they began to look sick? Has anything besides bluebells grown there before? Did they die slowly of fairly quickly? Have you used any weedkillers around the garden? Is the soil deep or shallow?

And another question to add to steragrams list: what time of year did you plant, and how long did the various shrubs survive?

My garden consists of 3 rows of 3ft x2ft slabs with 2 removed from the centre leaving the 4ft x 3ft plot.
The garden faces east & due to surrounding buildings gets the sun from 8.30 = 12.00. I agree that this is too small for 2 pyracantha's but I did not plant them together
but on 2 separate occasions. I'm not sure when I planted them but the were bought at a garden centre ready for planting along with 42 for my back garden which are all doing well. The 2 in the font garden lasted 2-3 months before dying off, & began to look sick. The hydranger was bought in July as a flowering pot plant but died off within days. The garden immediately next to mine is totally neglected unless I dig out the occasional sycamore. In fact it was this garden that I obtained the bluebells which have flourished. The garden on other side has roses in bloom. The soil is moist & deep & had a large bag of compost mixed in at the start. I only use systemic weed killer on the outside perimeter of the garden not on the plot

The reason I asked about the time span, and the timing, of planting and dying, is because this might be a water problem. Certainly, a hydrangea planted in July would have needed watering every few days, and if the weather was hot, possibly daily, with a good watering can full (without the rose on) distributed around the base. Any new shrub will need extra care until it has extended its own water seeking roots, and its possible that this area is more exposed to drying winds because its in the centre of the garden and east facing. The best time to plant a shrub, particularly an evergreen, is October, although in the North, September is good. The fact that the bluebells are surviving suggests there's nothing wrong with the soil itself, and being a bulb, these are able to withstand periods of dryness without too much trouble.
That said, Pyracantha isn't a particularly good choice for a specimen growing alone in the centre of a garden because it's not a tidy grower and prefers to be up against a fence or wall. Seems like a good spot for something like Kilmarnock Willow (Salix caprea 'Kilmarnock') or Caragana arborescens 'Pendula', though the latter is thorny and may get too wide for the area. Kilmarnock Willow can and should be pruned to keep it a tidy shape as it ages.

many thanks