There are several reasons why container-grown shrubs will not flower or stop flowering.
Drought
Towards the end of the growing season the wood of many shrubs begins to ripen and flower buds are initiated for the following year. Lack of water at this vital time can stop this development reducing flowering in the following season or causing bud drop. Ensure that during times of drought in late summer and early autumn plants are watered thoroughly.
Fill the container to the rim, allowing it to drain before filling a second time to ensure that the compost is moistened to the full depth of the container. Check for moisture daily from April to September, and evergreens and conifers weekly during the winter months.
Waterlogging
Excess moisture can kill plant roots due to a lack of oxygen and subsequent root decay. This will reduce the ability of the shrub to take up necessary moisture and food leading to a decline in growth and flowering.
Use containers with adequate drainage holes and raise plants up on feet in the winter. In excessively wet weather containers can be sheltered against a wall where they are less likely to become waterlogged.
Feeding
Most composts have sufficient nutrients in them to provide food for six to eight weeks after potting on. After this plants need additional feeding to maintain healthy growth. This can either be in the form of controlled-release fertilisers used once a year in spring, or more frequent use (from weekly to fortnightly) of liquid feeds.
Pot-bound
Root-bound plants will rapidly suffer from drought and lack of food. If this happens then they should be potted on to the next size pot. If plants are in the largest size pot that can be managed, top dress or remove excess compost before replacing it in same pot.
Ensure that such plants are kept well fed and watered, as they will be far more prone to drying out.