The clutches wear, just like any other clutch. They are designed to slip into gear gently which causes more wear. People fit shift kits to their auto to help speed up the shift, and put more pressure on the clutches to stop the slipping.
If the clutches are worn they are quite hard to replace, and usually the entire box is rebuilt.
It could also be a sticky or blocked solenoid or filter. Probably best to get an opinion from a respected local auto specialist.
The attessa has nothing to do with the box, other than being bolted to the back of it...