In my experience escape artist will always back out of a harness and fear usually is a big factor. My suggestions will be:
Adjust harnesses where one or two fingers fit under all straps
I double leash all new dogs or escape artist and using the harness for all tension. The collar leash is for backup
I also find short legged dogs slip right out therefore double leash will help for safety
Here are the two leashes I have had success with over the years (see attached link below). I know there are harnesses made just about every month but these are the two I always find that works.
It’s like a dance when walking a dog that escapes. Watch your environment for any stimuli that may make a dog want to escape and use your tools and training. Double leash until the dog is trained is the safest option IMHO.
Hipe this helps and feel free to ask questions at anytime.