Get yourself some Canna PK 13/14. Its supposed to be used at week 5-6 to really boost flowering. Its really powerful stuff and you only need to use it for a week. I would also be giving them some potash supplements to get your buds swelling. I still add a small amount of veg nutes (5ml to 10L) to my flowering mix, your plants will appreciate the extra nitrogen and it stops the bottom leaves from dying off slowly and keeps the plants productive. What is your current nutrient program?