We viewed your photos. With Japanese maple most problems are not insect or disease related. It has more to do with growing conditions and location. We notice some damage to the main trunk (cannot say why) which may have contributed to branch dieback. We also notice some cold damage (white areas) on the leaves. This most likely occurred due to late spring frosts and is not uncommon on these trees. There is not much you can do for this. 

We notice excessive mulch around the trunk. Make sure mulch is no thicker than several inches and keep away from the base of the trunk. Water the tree during dry periods. All you can do is scratch the branches with your fingernail and look for green tissue. If you see brown/gray, then it is dead. Prune dead wood. If this is in the main trunk, there is not much you can do. 

Here is our blog on Japanese maples and how to grow them successfully in your landscape https://marylandgrows.umd.edu/?s=japanese+maples

Marian

Also, it looks like your tree may be planted too deeply. It looks like a telephone pole going into the ground. This situation can contribute to trees having too much soil piled up around the trunk causing failure to establish, trunk damage, poor growth, decline, etc. A properly planted tree flares at the base of the trunk at the soil line where it joins the root system. If so, Carefully remove the excess soil or mulch from the circumference of the trunk to the point where the trunk flares out into root growth. https://extension.umd.edu/resource/trees-planted-too-deeply

Marian