Time outs only work when they are chosen by the child, and therefore do not work when they are too young to understand the reasons behind a time out. There are two effective methods of disciplining I believe - consequences that fit the action and time outs that the child chooses.
I have to assume some things here, so bear with me. Perhaps the place you use for time out is not welcoming. Perhaps your child is too young to understand what its all about. Time out space should be comfortable to a child - with a big teddy bear, some music etc, placed somewhere like a corner of the room, or a spare room if you have one. They work best (only?) like this -
Parent - "Ok, that behaviour is unacceptable to me. I really think you need to go and think about it. Why don't you go to your thinking space."
If the thinking space (time out) is welcoming, they may well trot off and do just that. Which gives them the opportunity to have quiet time and digest the whole event. I don't believe in punishment and I fully support choice, this gives them the choice. Time out should be useful and have a point, therefore it should be welcoming and comfortable - not akin to a jail cell where priveleges are taken from them (punishment).
And yes, I have had many an argument with other parents about this method, and yes, their argument is always "how does that do anything, if it is comfortable and welcoming and they would choose to go and sit in teddy's lap, where is the lesson in that?" to which the only response from me is "what is the lesson in punishment?" Keeping in mind that whereever possible, time out can be on you. My DD often uses me as her time out, I just say "thinking time, darling" and she either goes to her space, or throws herself on me and processes the event which usually culminates in her coming back to me/looking up at me and saying, "oh, sorry mama, I was just playing."
A strange and wonderful thing that occurs with a gentle time out space, is that sometimes you will catch them waltz themself over to it, switch on the soft music, snuggle on the big ted's lap and do some voluntary thinking. It takes time to introduce the space, and if it is delivered in a loving way, you may find your child will be more cooperative when you offer the suggestion she may need some time out.