I have no problems getting my sims engaged and married and all that jazz, and have done it numerous times. At least once when I had two sims get married the bride changed into a white wedding dress under the wedding arch and that's what she wore to get married in. However now no matter what I do, I cannot get that to happen. Now the sim I am trying to get married just changes into her regular formal wear for the wedding. How can I make her wear a wedding dress?