The last 2 cakes I have made have not turn out so good! I am using a mousse filling and no matter how I dowel it the cakes end up looking like a flat tire! I dont know what I am doing wrong?

Can you give us a little better description so we can identify the problem and help a little better... or show us a picture. That way we can get you help and going in the right directions!

Is the cake fondant covered and the middle buldges? With a mousse you need a good dam to hold it in. I learned the hard way, but plenty of others helped me out. Thicken up your icing enough to make a heavy bead around the edge of the cake you're filling. Then do another bead just inside that. Then fill.
You might try letting the filling set longer in the fridge (4-6 hours) and then filling.
When you put the top cake on, push down on it a bit to settle it (or set it aside for a bit to let it settle itself out)... otherwise the weight of the cake on top will settle later after you put your fondant on.
I used wood dowels forever but they are a pain in the butt. I started using tea straws and I love them! For one, don't over-dowel a cake, you ruin the integrity of the cake by shoving too many dowels in. 4-6 tea straws in plenty (as long as it's not a 7 tier cake... then you'd need more support). I love the tea straws because you can stick them in the cake and cut them flush with the surface easily with scissors. Once a dowel is inserted, you don't want to remove it, so tea straws are awesome for that.
I hope this helps, I have been told the same info froms some of the greatest on here... hope I can help pass it along.

yes thanks so much! i have posted some pics ot the two cakes im talking about. its the zebra /lady bug cake and the my lil pony.i usually ake my fillings a day or 2 aheado of time but i probably did need a better dam! thanks for your help...oh and what ware tea straws?

I had a look at them and they don't look like they buldged too bad at all. You want to see buldging, you should have seen mine! haha!

Yeah I would say if you beef up your dam and it should help a lot. I put the dam in, then fill and look to make sure the filling is level and not above the dam or at least pulled thinner toward the edges so when I squish the top cake down it has room to move out towards the edge without overtaking the dam. Give a good press on the top cake so you know before your fondant goes on if it's going to break the dam.

Tea straws are just regular drinking straws, but heavy duty... like you get at Starbucks or McDonalds milkshakes. You can cut them with scissors easily and they are really sturdy in their column shape. Like I was saying, I use 5 tea straws in a 12" cake to support 2 layers above it.

Good luck! Let me know how the next one turns out!

Thanks so much! I have another one next weekend I'll try the straws and dam on that one and let you know how it omes out!