It sounds like something is stressing your bougainvillea plant. This is usually due to something damaging the roots. Drought, soggy soil, damage from a weed killer used nearby or digging around the roots are all possible causes. I suggest you try fertilizing it and then water it well but don't keep the soil soggy wet. If there are weeds around it pull them and give it a mulched area to keep the weed competition away.
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