Try keeping the potting soil evenly moist to slightly dry. Letting the soil go through extreme wetting and drying can result in leaf edge burn and poorly developed flowers or bud drop. Regal geraniums (martha's) don't need as much fertilizer as zonal and ivy geraniums, so that may be a factor in drying/dropping flower buds as well. You may want to move it to a shadier spot or insulate the pot .... good luck!
Debbie

Thanks Debbie! I have been feeding them all at the same rate, so I'll start cutting back and see how that goes.