Potassium is the primary nutrient that helps them swell, but when they swell.. they need more phosphorous.. carbs help too, .. <insert molasses promotion, use molasses when not using a bloom fert!!>
From the sick plant guide...;
Phosphorus does a lot of things for the plant. One of the most important parts of Phosphorus is: It aids in root growth and influences the vigor of the plant and is
one of the most important elements in flowering as well helps to germinate seedlings.
Phosphorus is an essential plant nutrient, and since it is needed in large amounts, it is classified as a macronutrient. Phosphorus is a MAJOR important nutrient in the plants reproductive stages. Without this element the plants will have a lot of problems blooming without proper levels of Phosphorus.
Potassium plays a big role as well. Having good amounts of potassium in your plants helps in having sturdy and thick stems, disease-resistance, water respiration, as well aids in photosynthesis. Potassium is also found in the whole plant. It is necessary for all activities having to do with water transportation. Potassium is necessary for all stages of growth, especially important in the development of buds.
Bolded the important part for you. Lighting is equally important as without good lighting the photosynthesis process goes to shit. Don't feed too much nitrogen during bloom as this will reduce the density of the buds as well.
