The leaves on my potted plant dropped off and are slow to grow back, it looks very spindly, I'm still getting blooms though.
Check the plant for loopers, they can devour the leaves quickly on a Bougainvillea.
Here is a link for what to look for.
Over watering is the most common reason for leaf drop. They do not need a great deal of water so make sure the soil is drying between watering.
If watering is not the issue, check for Aphids. They too, are commonly found on Bougainvillea and can be treated with Neem Oil.
Lack of sunshine can also cause leaf drop. Adjust the light if your plant grows in a pot.
When the weather is cooler your plant will also drop leaves.