Dog Rose Not Blooming

I purchased one of your dog roses about three years ago. I don't have a problem with it growing, as it comes back every year and grows like a weed. The problem is that I have not had one bud on it in the three years that I have had it. Can you give me an idea as to why it may not be blooming?

Most species roses and the dog rose in particular flower on the previous year's growth. Also the plant has to be mature before it will bloom. This is especially true of seed grown plants. Plants grown from cuttings or grafts will have the maturity of the parent plant of the graft or cutting and therefore bloom much earlier. Dog roses will be about three meters tall at maturity. If you want blooms you must never shorten the canes you want to bloom.
Also if the plants are grown with too much nitrogen they will not bloom. Therefore make sure that if you use manure as fertilizer that it is well rotted and maybe add a bit of wood ashes to increase the phosphate content. The soil should be neutral to slightly acid.
Roses must have full sun to bloom properly. A slight deficit of sun can be offset by a high phosphate fertilizer (high middle number). These fertilizers are often called something like "blossom booster" or "flowering" fertilizer.