I had an Acer(I am using shorthand here for Japanese maple) that flourished growing underneath a large oak tree. What I have just said gives you a hint of what the Acer likes in terms of growing conditions and that is part shade. Now that is what has to be known....your growing conditions for this tree....so members can give you a good informed answer to your problem. So add a bit more detail to the surroundings of your tree such as ...type of soil, drainage of water when it rains, lighting conditions, what is it growing near..etc. Some photos of your Acer would be of help with distant and closeups of the trunk, branches and a leaf or two so members can check the tree for infection and or infestation. As it stands now it's like driving your car to the mechanic because something is amiss with the engine and asking the mechanic to give you an answer as to what is wrong with it while having the hood closed. Best Regards and Welcome!:)

Also, and very important, is your Acer planted where it is windy? If it is the leaves will never look great. Our are, in general, in pretty much full sun as too tall to be under anything but they leaf up beautifully each year. Also are your Acers in the ground or in pots?

Are they in pots or in the ground is critical to know, as well as whether they're in sun, shade, a windy spot, etc.

Hi Cazzy and welcome to Goy...my friends above are absolutely right. You haven't given us much to go on. But Acers need to be away from strong winds, in good well-drained soil but have plenty of water. They shouldn't receive sun before 2pm. Given these conditions they should do ok. So check these things first and if that doesn't help, please come back to us with some photos! :)