Under pines, in shade, not much water.... I think that is your answer. Spiraea 'Goldflame' etc need a sunny spot in moist, well drained soil. They can be renovated by cutting back hard after flowering. But it sounds to me as if yours are just declining from poor growing conditions so pruning won't really help.

"Under the pines" and in "dappled sun/shade" sounds like the bulk of the problem. While these shrubs can take some light shade they are really recommended for a full sun (6+ hours) position. And it is quite likely the pines are hogging some water as well.

I'd consider moving these to a better location. Spiraea is a tough plant and can take some pretty challenging conditions but the better you provide for it, the better it will perform for you. More sun, consistant watering as needed and good drainage and you should be rewarded with full, vibrant plants that bloom heavily.

i have moved them in august .... which is usually a big no no ...

they are nearly unkillable ... with a little proper water aftercare

and can nearly be cut to the ground...for rejuvenation ....

what do you have to lose.. besides some excersize ???

ken