Acers seem to have minds of their own! We have three, a 'red' one, a 'golden' one and a 'green' one, all in pots. Have had them for years. They fared best in our previous home, West facing but under a large tree so shaded for part of the day. Never a leaf drop. Here they are struggling, can only have them East facing in a fairly windy garden. The red one hates wind and drops leaves as and when it feels like it, the other two are faring better but are struggling without the shelter of the larger tree. I'm no expert but wonder if yours might benefit from some protection from a larger bush or tree if that's possible? Ours certainly liked it. We did have a new one - it died - discovered it was waterlogged, they obviously don't like that! Not sure about compost, ours are in ericaceous, I feed them now and again with a feed for rhododendron/azalea/acre type plants. Hope they recover, I know how frustrating it can be, they are beautiful when they thrive but a right headache when they don't.