the bud opening is an act of the flower drying out. when they are fresh and wet the stay rolled up. when they start to dry out some they begin to open. You need more airflow maybe? seems like you have plenty
more light seems like you have plenty of that too. should open it right up and dry it to opening stages if its getting the water it needs..the top buds are always the very last to open also and the water struggles to get to it.
Now watering. if the fresh little bud isnt getting enough water to fill the cell walls of the flower into maturity its not going to open. humidity provides an outside source of moisture as the plant is using most of it the water you are giving it. A little more water maybe? Try using warmer water instead of cold water out of a bucket..warmer water travels faster and is easier on the plants than cold. I know you dont use COLD water but try warming it up some in the sun before applying.
fertilizer. you dont fertilize flowers but the plant does need potassium and phosphorous in which to make good strong blooms.
and your like wait a minute it needs to dry out but need water. uh uh its the transfer of the energy from the watering and drying process that will open it up. Plant aerobic exercises.
sorry Ive been really yakky
trying my ideas out to see who disagrees with me mostly