Hi I too have the Ezy-Dog harness
and Yes my boy Darwin shimmied out of his too.
But then he's got out of EVERY harness we've bought for him
So we now use the lead doubler method.
first you need a police lead
you can get these from Indi-Dog too, superb quality (link is on the forum main page)
then you need the doubler
Clip one end of the doubler to the collar, thread the doubler through the handle of the harness to the police lead and clip to one of the lower rings
Clip the police lead to the big D ring on the Ezy-Dog harness as normal, and you're done
Now when he shimmies out of the harness he's still connected via the collar to the police lead.
this gives you time to stop and get him back into the harness.
My boy Darwin used to escape ALL the time.
He hasn't got away from this set up in two years.