Your roses probably are needing to be fed and watered a little more to create stronger growth and more foliage. It is also not unusual for bloom size to decrease in the midseason. They should be pruned back just a little in late August in most years and will then read grow strongly in bloom quite well again in October.
When Gardenia leaves turn yellow it is usually a sign of a lack of nitrogen or any soil water problem. Both too much and too little water can damage roots and cause the older leaves to turn yellow. If the new leaves are turning yellow that is a sign of iron deficiency. You will need to purchase and apply an iron supplement to maintain a good green color to the foliage.
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