I had the same problem with the harness from GW Little. We bought it for Lola and had her name printed on it only to realize that she is able to slip right out of it!! I looked for a long time for a harness for her when she walks so that it doesn't pull at her neck and for when she is the car seat with her brother Tino so she is completely safe. I ended up taking a chance on the Puppia step-in harness and it is GREAT! We ordered her the XS and it only comes in maybe one or two styles...but I would be happy to give you the link from the ebay store we got ours.
Try here:
eBay Store: Search results for My Doggies Closet.
Good luck