try eating a few thousand more. in other words, you can have the "best" plan for yourself at the time but still not gain due to lack of food. and the best plan is so subjective as belief in a program means a lot when it comes to success of a plan. and the best plan changes as your training age increases. there are proven programs that help you to adapt the plan to your needs. get on one of those. me being a strength athlete i dont follow the mass plans.
as for eating so little over your maintanence, do you take into account the daily change in caloric needs? some days you may burn 500 more calories and therefore not gain much. in other words, eating more always helps. not gaining, eat more. not gaining still, get on a proven plan for mass and eat even more. when i want to gain strength i eat when im not full, not just when im scheduled or hungry or when i can.