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If you have not pruned any, it may be in need. Pruning can help rejuvenate the shrub and improve its overall health and blooming, as well as its appearance. You can do this once flowers have faded out by one third. As for fertilizing, it generally needs little, other than an occasional all-purpose rose-type fertilizer. If blooming still seems to be an issue following pruning, your soil may be lacking phosphorus. Add some phosphorus, such as bone meal, to the soil. This article should help as well: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

It may be thrips. They will attack flowers or new growth. This article will help:
https://www.gardeningknowhow.com/plant-problems/pests/insects/controlling-thrips.htm

Neem oil may help as well.

This could be due to a nutrient imbalance in the soil - such as high nitrogen and low phosphorus. Try adding some bone meal to the soil to bring up the phosphorus levels and encourage blooming. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

I would suspect this is due to low temps. Has it been cool where you live? It could also be due to the wonky winter weather most of us experienced. With mild temps across much of the country, many plants and trees budded early. Some of us were then hit by unexpected cold spells, which may have caused considerable damage to the buds--with many failing to open or those that do are either deformed or not as fragrant.

If you have not pruned in a while then it may be in need. Pruning can help rejuvenate the shrub and improve its overall health and blooming, as well as its appearance. You can do this once flowers have faded out by one third. As for fertilizing, it generally needs little, other than an occasional all-purpose rose-type fertilizer. If blooming still seems to be an issue following pruning, your soil may be lacking phosphorus. Add some phosphorus, such as bone meal, to the soil. This article should help as well: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

When flowering plants don't flower, the first thing to look at is light --- it may need more. Also, it may be overfertilized with too much nitrogen. Try just working bone meal into the soil instead. It might also be too dry --- try mulch to retain moisture around the roots. Here is more information on growing mock orange: https://www.gardeningknowhow.com/ornamental/shrubs/mock-orange/mock-orange-bushes.htm