Did the venders say why they couldn't fill those tanks? Did they have the correct fittings?
My guess, is that the safety valve in the valve assy is malfunctioning (or maybe functioning fine, because there is a defect)
DO NOT try and make this repair yourself, unless you want to have a shortened life! LP isn't really something that should be messed with....especially, if you don't really know what you're doing.
What I would do, is to "Swap" the tanks at one of those places like Home Depot, or Walmart, and just get new bottles......you can then have those refilled when you need to, without having to swap them again.