Most retail orchids are epiphytes, normally growing in the branches of trees. To mimic this situation, they are potted in very light, airy mixes. These potting mixes don't have much in the way of nutrition, so to help the plant build up enough energy to bloom you must feed it.
In the winter, orchids are less active and less needy. Once spring arrives, the orchid starts its growth efforts and needs to be fertilized. Brennan's Orchids website recommends feeding your orchids every other watering, spring through fall, with a balanced plant or orchid-specific food.
Miracle Grow is a fine product but is not specifically formulated for orchids. You can use it successfully on orchids if you dilute to no more than half the usual strength. Orchid roots burn easily and this protects them from damage.