There are many reasons that a plant may not thrive, or it may thrive then get sick. Plants like people, have a variety of tastes and needs.
Plant health factors
Here are some things to consider when diagnosing plant problems.
Is the plant in the right place and does it have the conditions it needs or avoid the conditions it can’t handle? Eg, sun, shade, soil type: clay, sandy, loam, damp, dry, wind.
Is it getting the right mix of nutrients (nitrogen, phosphate, potassium and trace elements?) for its needs?
Is the soil washing off in the rain and taking nutrients with it?
Is the plant getting enough, or too much, water?
Are there visible pests? Eg insects, eggs, chewed leaves, silvering leaves, distorted growth.
What about pathogens, viruses and bacteria? Mildews, rusts. blights, galls, dieback?
What is happening in the soil? Pests such as grass grubs, unhealthy bacteria etc.
Do you know what is under the garden itself? If underneath is clay, compacted soil, concrete or rock-fill, plant roots may not be able to get the water and nutrients they need. There could be contaminated soil from long ago.
See Repairing the soil for a good place to start bringing your soil back to life.