I was not even sure how to search for this in the forums. So I am just asking out right.

I have a silver bastard sword that cannot be improved like other weapons at a armorer's station.

This silver bastardsword is a exact copy of the 1 handed silver sword but then I changed the number of hands (2h) and raised damage and weight a little, in the stats tab and the models in the arts tab.

see here: http://spookyfx.com/Skyrim/SilverBS.jpg

But that is all I changed, the keywords are all the same and there is no special enchantments or scripts. So why would the weapon be unable to improve (raise the damage it can do) in the game?

Maybe the fact that it's silver and a greatsword? Isn't a sword the only silver weapon in the game? Maybe it's handled uniquely. Try looking at the enchantments themselves that act as the 'sharpening' buffs. See if there's a difference from the standard ones like iron.

Did you make a tempering recipe? (http://www.creationkit.com/Bethesda_Tutorial_Customizing_Weapons_%26_Armor#Vorpal_Sword:_Tempering_Recipe)