Acer - Japanese Maple

I recognise this. I'm sure it's simply dryness at the roots. These japanese maples have shallow roots and are prone to drying out, which causes the leaves to shrivel. Unless they are planted in a shady, damp, sheltered place they really need a bucketful of water once a week to keep them happy. If you do that they can even cope with full sun and a bit of wind too.