Overall, the most common reason that orchids fail to bloom is insufficient light. The Phalaenopsis and Paphiopedilum orchids are usually happy with the filtered light in a windowsill but many other varieties need more light than that. Dendrobium, Cattleya, Oncidium and other medium to high light orchids may need supplementary grow lights
or time spent outdoors in the summer to get the kind of light levels they require to bloom. Orchids are not like tomato plants, the vast majority cannot handle full sun. Dappled shade is ideal and even though a bright window seems like more light than outdoor shade, it is not.
As an orchid gets more light its leaves turn a lighter shade of green. Very light yellow-green leaves usually indicate too much light where very dark forest green leaves can indicate too little light. In summary, if you have a Dendrobium, Cattleya, Oncidium, Cymbidium, Vanda, Brassia or other high light orchid growing indoors on a windowsill and it has not bloomed in a year or two, chances are a lack of sufficient light is the reason.
One approach is to take orchids that require high light and put them in a vanda basket hanging from a tree branch. Higher humidity supplemented with sufficient watering allows high light orchids to have a vigorous growth season. We've seen Dendrobiums rebloom with this kind of treatment even after they have languished without blooming on a windowsill for ages. The key to getting the bloom was more light.