You may be using too much shampoo, and you should not need conditioner. You may have to find a new shampoo that is more agreeable or tolerant of soft water. Your hair is cleaner now when washed in softened water than it is when washed in hard water. It is a rare thing for someone to mention a hair problem after installing a softener but... your hair should have less 'body' to it when it is cleaner because of having no hard water caused shampoo residue on it. That may be the "flat" part and usually your hair will be less likely to stay in place because it is 'wispier' and will blow around easier than when washed in hard water.