When I take pictures in the room with A 5.6 and flash mode TTL BL, why the results are quite dark?

Could you please help me? Recently I took some pictures in the reunion with A 5.6 andflash TTL BL, but I am very disappointed cuz the pictures look quite dark. So I have to adjust them again in the photoshop to make them brighter . Do you know any sofware to improve the quality of pictures like GEM or corel Paint Shop Pro X?Thank you