Shrubs grown in containers often become root bound. A shrub's roots must grow in order to accommodate their growing foliage. When there's no room to spread outward, the roots wrap around the interior of the pot and become "bound." When this happens, these needed extra roots cannot absorb the nutrients they need. Consequently, the shrub may not have enough nutrients to bloom. To encourage flower production, you must score the root and transplant it to a pot that provides sufficient room.
Water the potted shrub thoroughly a day or two before you repot it. Hydrated roots tend to recover more quickly.
Remove the shrub from its current container. Gently press on the sides of flexible plastic pots to loosen the soil. Lay the container on its side, grab the base of the shrub and gently work the root ball back and forth while pulling the shrub out of the container. Do not force it. If the roots are severely root bound, you may have to cut off or break open the pot to free the shrub without damaging the stem or the roots.
Score the shrub's roots. Take a sharp knife and make four vertical, equidistant slits. Each slit should be 1/4 inch deep and run from the top to the bottom of the root ball.
Loosen the shrub's roots by gently pulling them away from the root ball.
Repot the shrub in a pot that is at least twice the size of the shrub's original pot. Fill the pot with enough quality potting soil (suitable for your shrub) so the top of the root ball sits approximately 1 inch below the rim of the pot when placed inside it. Then finish filling the pot with potting soil so no more than 1 inch of soil covers the top of the root ball. Pat the soil down to remove air pockets in the soil.
Apply a root growth stimulator to the soil according to the manufacturer's instructions. This is not an essential step, but it will stimulate your shrub's roots to grow and recover more quickly.
Water the soil thoroughly.