Information about your mock orange can be found on the usda website: http://plants.usda.gov/plantguide/pdf/pg_phle4.pdf

Mock oranges (Philadelphus species) are deciduous flowering shrubs named for the orange-scented white blooms that can appear between May and July. Mock orange is adaptable to most soil conditions, although it prefers to dry out between waterings. Most problems associated with the mock oranges are moisture- and insect-related although they are susceptible to a variety of bacterial and fungal diseases.  These include Gray mold, a fungal disease caused by the fungus Botrytis cinerea, as well as other species.  While gray mold usually inhabits dead plant tissue it can invade stressed or weakened parts of its host plant or affect healthy plants at wound sites which are prone to infection. Gray mold can appear on any part of the plant especially in humid conditions, and appears as gray-hued web-like substance.  If you have gray mold remove the affected parts of the plant ans destroy them far from the plant – also remember to sterilize all of your tools.  Another fungal disease which affects mock oranges is Nectria twig blight, caused by the fungus Nectria cinnabarina.  It appears as areas of dead tissue called cankers which then develop reddish=brown growth.  Nectria twig blight is associated with excessive moisture.  This is not a life threatening disease however sick portions of the plant should be removed and tools sterilized.  Spots on the leaves that appear black or brown and look saturated with water are a symptom of bacterial blight,caused by the bacterium Pseudomonas syringae.  Bacterial blight most often attacks mock orange trees when weather coosl and moisture is high. It causes dieback of shoots and leaf deformation. Once again removal of the diseased area is recommended and copper-based chemicals can be sprayed on according to directions to kill off the bacteria. Powdery mildew is a common fungus which tends to develop when the days are hot and humid and the nights are cool and humid. It appears as a white dust on affected leaves. Prune off affected areas, spray with copper fungicides or sulfur, water in the mornings and avoid overhead watering. Make sure your mock orange is in a place that gets plenty of air movement.   You said you did not observe bugs – the ones to look for on your mock oranges would be aphids, tiny insects, which can take on any number of colors hang out on the undersides of leaves and suck the sap.  Plants with aphid damage tend to have yellow or curled leaves, stunted branch growth and may exhibit black spots caused by honeydew excretions. You can turn your hose on your bushes and wash the aphids off the leaves in the morning or treat with an insecticidal soap.  The other common bugs found on mock oranges are leaf miners, which is a general term for larvae of several insects, such as flies or beetles, that lay eggs on the undersides of leaves. When the eggs hatch, the larvae eat the leaves, leaving holes. Look for holes and furrows in the leaves, browning or curling leaves and black specks on the leaf surface.  Cut away the affected stems and apply insecticidal soap.  Remember to move the leaves distant from the plant and clean your tools well. Your mock orange should be planted where it gets light shade in the afternoon and it needs well drained soil. They like to be well watered during their growing season and don't normally need feeding, however, you might want to consider giving it a good rose type fertilizer early next summer. Mock oranges do best if they are pruned regularly so yours may just need a good pruning; cut back the oldest, outer stems, remove any sucker growth and prune for shape. Making each cut just above a strong, outer facing bud or new shoot.