are you keeping your ammonia at? Are you testing & continually dosing your ammo to maintain consistent level? And, be careful of overdosing.... one good overdose of ammonia can pretty much nuke your bacteria and cause it to start over. This is why I prefer the dead shrimp method... drop a couple in for a 55 & let them rot. No worrying about maintenance doses of liquid ammo.
tank is different.... While is seems exceptionally long, I've had tanks take 3 months to completely cycle.