The concept of sail balance is pretty easy to grasp. Powering up the headsails will produce lee helm as it moves the Center of Effort (CE) forward. Powering up the mainsail will produce weather helm as it moves the CE aft. When the forces of the main and headsails equal each other, the CE will align with the Center of Lateral Resistance (CLR) and will sail straight.
If you suffer from weather helm, all you need to do is add more headsail area and/or reduce mainsail area, as this will move the CE forward and reduce the weather helm felt by the captain. A genoa is a massive headsail which should give you plenty of leeward force and help balance out the yacht, but this may not be the case.
As you sail and feel weather helm, you might try reefing the main and letting out more of the genoa from the roller furler. Somehow, nothing seems to have changed! The weather helm is still present. Letting out more genoa doesn't seem to help any more either, as the problem continues to exist!
Why would such a large headsail fail to balance out the sailplan? The answer is simple: a genoa is a jib that extends beyond the headsail triangle and continues aft of the mast. While the headsail is set forward of the mast, the portion that exists aft of the mast will move the CE aft. The larger the genoa, the more this effect increases.
This is why you will see people beating to windward under genoa alone, with no mainsail, and they will be sailing along at a good pace! The large headsail, especially with genoas that are 150% or larger, will create enough weather helm by themselves that they are able to power your yacht to weather. Adding a mainsail to the combination only exasperates the situation, adding even more CE aft which will create even more weather helm.
If you have a genoa and find your self struggling with weather helm, consider roller reefing your genoa before you try to reef your mainsail. Reefing your genoa will actually move the sail area forward and also move your CE forward which will help reduce weather helm and balance your sails.