Could you send some pictures to peterb@hewitts.com

The first thing I’d suggest is checking the base of the plant to make sure that they weren’t buried too deep or that mulch hasn’t been piled up against the trunk.  Get under there and brush away and mulch or dirt until you find the original soil that the plants came in.  Brush back the soil several inches away from the trunk and keep it that way.  Soil or mulch piled against the bark hinders the flow of moisture and nutrients up through the bark and will kill the arb (or any shrub or tree.)