Your question understandably contains little information in order to give you a specific answer.

However your question does seem to indicate you understand the game quite well and are aware of the many variables within it.

If I am reading correctly there is potentially a clue contained in your text, if I am wrong then it still doesn't rule out the possibility of it being a possible  explanation.

Are you the attacker or the defender, presumably on a tile  rather than on the town?

Are there definitely no other kind of troops involved in the battle?.

Siege are the strongest against melee so  its understandable that you think you should win.

You do have a lot of  oracle knowledge,but is it the optimum  for these battles ?

 You could be falling into traps,otherwise if you are being attacked, you could be scouted first ,and then the troops sent  could be optimised in their favour. It could be as simple as numbers sent, or  use of other  game features.

Only 100 billion influence for Aesir/hero/buildings.

Your Shrine of Odim must be maxed - the number of troops you can send on a march is critical.

Maxing some of the Aesir less critical but a help.

At your size I would expect a maxed hero, well advanced Einar and a full set of legendary special equipment for hero and shaman - have you put enough time and effort into that aspect?

If you got 500 knowledge, then you got like everything. Secret knowledge, aesirs, all troops in citadel etc huh?

obviously you want the best gear, top level hero and shaman. Max out town skins, secret knowledge and so on, i have 300b knowledge. I have 4 troops maxed in citadel, nearly 2 secret knowledge complete. 300/400, vip level 39.

You simply need the best of the best. A 200b town can pretty much have all these things for 1 troop type. Going further means more influence.

 A 200b town can pretty much have all these things for 1 troop type. Going further means more influence.