The most common reason for failure to feed is because the reptile isn't set up with the proper environment. Environment is the specifics of lighting, temperatures, humidity, furnishings, size, design and layout, and photoperiods. If any of these things are wrong for the species, it can cause reduction in appetite or a complete failure to feed.

The most common environmental problem is wrong temperatures, either too hot or too cold. If you don't use a thermometer, use one. If you have a thermometer and it says that the temperatures are where they are supposed to be, get another thermometer and double check. Sometimes thermometers fail, or are less than accurate to begin with. If you have the self-adhesive type of thermometer commonly sold for reptiles, and have peeled it off and re-glued it to another place, that can destroy its accuracy. Heat-sensing guns can be used to check the heat at places difficult to place a thermometer, such as on a branch used for basking by an arboreal reptile, or rocky perch used by a desert lizard.

All of this assumes that you already know what the proper temperature gradients and basking temperatures are for your reptile. If you don't, find it out! Every species has different requirements. While they can be loosely grouped by habitat type (woodland, desert, wet tropical forest, temperate riparian, etc.), where they live in that habitat can affect temperature requirements, as can their lifestyle (nocturnal or diurnal). If you can't find species-specific information for that rare swift you have, research the requirements known for other species of swift and other lizards that live in the same places as they do, and compare what is known about yours (habitat type, lifestyle, habits, etc.) to what the known species require.
You need to provide heat in a way that is similar to how the reptile obtains and uses heat in its environment.

Very few reptiles actually bask on hot rocks in the wild, so, despite the advertising of such products, hot rocks are appropriate for very few species of snakes and lizards, and not at all for chelonians. Humidity is an important environmental and health factor.

If the environment is too dry for the species requirements, it will cause dehydration in the reptile. Dehydrated animals reduce their food intake until they simply stop feeding. Forcing them to eat only worsens the dehydration as the gut pulls fluids out of cells to try to get the food digested. Serious, even fatal, hypovolemic shock thus can be induced.

Lizards and most chelonians require ultraviolet B (UVB) wavelengths in order to manufacture their own vitamin D3, which is a critical part of the calcium metabolization process. This is especially critical for herbivores or largely herbivorous reptiles, as well as diurnal insectivores.
The only safe ways to provide UVB for your reptiles is by giving them access to direct sunlight (not filtered through regular window glass or reptile/aquarium tank glass, or plastic) or by using a UVB-producing fluorescent light made for this purpose for reptiles.

Plant and aquarium lights do not produce the UV wavelengths reptiles need to produce D3.

Full-spectrum lights may emit the full visible spectrum (many don't as they are "color corrected" to affect the colors you see, which affect the way your reptile's colors appears to you and your reptile's environment appears to the reptile), but they do not - cannot - produce UV. UVB-producing fluorescents must be properly installed (no more than 18" from the reptile) and replaced at least annually. While there are several lights made for this purpose, not all produce enough UVB.