Autumn colour on leaves varies year on year depending on the weather conditions - sharp frosts arriving before leaves have fallen means loads of autumn colour - a slow, mildish slide into winter produces less satisfactory ranges of colour.
You don't say which Cornus or Euonymus you have planted - E. alatus usually colours up pretty well regardless, but most of the Cornus aren't terribly spectacular in autumn.

The only shrub cornus I know that has good autumn colour is C. Alba. There are others in the tall shrub/small tree section that give autumn colour. Cotinus is not one I know much about except the more common purple leaved variety of 'smoke-bush' - again, a small tree rather than a shrub?

Are they pot bound in the soil - heavy clay can effectively recreate a pot in the ground. I lifted a Spirea the other day and it was pot bound still after a year and had sitting water underneath and that was after I had tried to break up the soil and improve it with bark chips.

Following bamboo comment - A euonymus alatus I planted in the summer has turned the most wonderful barbie pink - quite spectacular in a bed of green and brown. Grows to 1.5m, not shrub shaped but midget tree shape. Please give us more info on plants and site.

Kildermorie - recommend wood chip or mulch (chip with leaf) is better than bark chips for breaking down soil. Also a fantastic weed blanket - ground elder when smothered just comes up beautifully in long strands as it is shallow rooting as many weeds are and there is no 'soil' to hold the root. As the worms take the chip/mulch down they create that lovely soft deep soil we all want - suggest you plan for good 4 inch layer to really break out top 12 inches of clay. Do you know a local tree/hedge surgeon, with a chipping machine, who is happy to sell or get rid of a load?