Thank you for contacting the UVM Extension Master Gardener Helpline with your question.

You highlighted 3 problems with your potted Gardenia that you have had for 3 years: leaves yellowing and dying, web-like structures on some foliage and lack of blossoms and flowers. Although a heavy manifestation of spider mites may account for the findings there may be more than one process contributing to these issues.

Gardenias are susceptible to a variety of pests including spider mites, aphids, whiteflies, thrips and root nematodes. Spider mites can cause yellow stippling of the leaves and with heavy manifestations can contribute to leaf loss. You have noted webs which can be a finding in spider mite infestation. You do not describe some of the other manifestations of spider mite infestation. Close inspection of the foliage, including the underside of leaves may identify other findings to suggest a pest infestation. Placing the plant in the shower and rinsing the leaves (including the undersides) may help to remove pests. (See references for management of spider mite infestation.)

Other factors may be contributing to leaf yellowing and lack of flowering. Yellowing of the leaves can be secondary to stress such as lack of nutrients, improper watering, over
fertilization, pest infestation, insufficient light, cool soil temperature. Yellowing of new leaves suggests micronutrient deficiency (such as iron) or root dysfunction. Improper soil pH can make it difficult for the plant to use nutrients resulting in nutrient deficiency. Soil pH should be between 5 – 6.5. It sounds like you are fertilizing the plant correctly - only during the growing season and using a fertilizer for acid loving plants.

Gardenias like bright light but not direct sun. It sounds like you have been supplementing
the light for your plant during the winter months which makes lack of light less likely as a contributing factor to the present issues. They are temperature sensitive preferring temperatures of 65 – 75 degrees Fahrenheit during the day and 55-60 degrees Fahrenheit at night. The plant should not be located near a heat source, or near windows or doors
where there can be potential drafts and lower temperature. You did not describe your watering schedule. Gardenias like humidity which may be low inside during the winter heating months. You can provide humidity by placing the pot on a saucer of wet pebbles. You don’t want the plant to sit in water as Gardenias like well-drained soil. Spitzing the leaves
with water is another way to provide moisture.

You say that the plant has not flowered and you do not see buds. Bud drop can result from
the plant being too dry, nematode infestation of the roots, too much fertilizer, cool temperatures, or poor drainage. I am not sure why you would not see any bud formation. I am including a collaborator in my response who may have some additional ideas about lack of bud formation.

https://content.ces.ncsu.edu/insect-and-related-pests-of-shrubs/pests-of-gardenia

https://edis.ifas.ufl.edu/publication/EP338

https://content.ces.ncsu.edu/twospotted-spider-mite-2

https://extension.umn.edu/news/managing-spider-mites-houseplants