The whole subject of fragrance and roses is very interesting and complex. In the last few years, a lot of work has been done to analyze the fragrances of many roses. A fragrant rose is more, or less, fragrant at different times of the day. Heat and humidity also impact fragrance.
[From The Rose Garden, by William Paul, p. 7: Pliny in his Natural History] tells us that the genuine Rose is indebted for its qualities to the nature of the soil, and that Roses without smell he does not consider genuine Roses...
[From Roses, by Susan Bales, p. 9:] The perfume of some roses changes as they open and age. Each rose fragrance is made up of a combination of compounds present in the oil of the rose. Some compounds evaporate more quickly than others and, as they do, the rose's scent changes -- many modern roses are scentless... The pink and red roses are the most highly scented while white and yellow roses are rarely fragrant. As they dry and wither, most rose petals lose their perfume while holding their color. Only the 'Apothecary's Rose' is known to hold its fragrance after drying. With the musk rose, fragrance is in the stamens. With the eglantine rose, most of its fragrance is in its foliage.
[From Roses, by Susan Bales, p. 10:] Fragrance is most pronounced and travels fathest on days that are warm and moist. The fragance decreases on cold, cloudy days or on very hot and dry days. Fragrance flourishes following a summer shower and after a light frost.
[From Botanica's Roses, p. 81: in a discussion of 'Angelique'] There is little in the way of fragrance, as is normal with good cut-flower roses, because the petals of such varieties need to be hard so that the flowers will travel safely to market without bruising; when petals are hard the scent glands function poorly. The myth that roses have lost their scent probably arises from the commercial necessities of the florist trade, although thousands of garden roses have retained their fragrance.