Specific insects or diseases tend to attack specific plants, rather than a number of different species. It could also be that one problem species is located in the wrong spot or just is not adapted to our climate. Problems affecting most or all yard and garden plantings, such as a vegetable garden or flower bed, tend to be related to the general environment or growing conditions provided. This includes the soil, amount of available light, watering, weed control, or perhaps the weather. Soils are a critical part of the success or failure of the garden.
Conditions related to soils include soil pH, fertility, and drainage.
Soil fertility is controlled by factors such as pH, organic matter, planting history, and fertilizer application. If everything is doing poorly, perhaps the fertility is too low or the drainage is poor. Too much shade is a very common reason vegetables or perennials don't do well. Do some pruning or consider moving the planting. Choose shade-tolerant perennials, groundcovers, and shrubs.