Check for mold on buds
When buds get big and thick, they become sensitive to mold, especially when they are dense. A good ventilation with sufficient extraction is the key to keep a decent relative humidity in the grow room.
At the end of the flowering period,look regularly around and inside buds. The usual mold attacking the buds is a botrytis, but you could come across many different mold.
If you find mold, remove it immediately and try not to spread spores from what you removed. Ventilate the room as much as possible using proper extraction and fan. You can trim a few big leaves to lower humidity and help ventilate.
Do not use herbs that have mold, it might be harmful.