Why didn't Edward affix his mother's soul to the suit of armor? If Edward could affix his brother's soul, why not his mother? Did it require seeing the 'truth' to do so? Does that mean the other empty armor suits (Barry) had another person who had seen the truth affix them?
Edward couldn't affix his mother's soul, because she was already long dead. It isn't made so obvious, but actually a few years pass between the mother's death, and then the boys training with Izumi, before they try and resurrect her. He was able to attach Al's soul to the armour because it was still present, in the body of the creature they tried to transmute. The mother's soul had long since departed, and couldn't be used in alchemy. 

When the mother died of her illness, we can assume Ed never considered putting her soul into another object, and even if he did he wouldn't have known how, and probably would not have had the alchemical skill to do so anyway.