These often become diseased and die back or die. What to do with yours depends on what, specifically is wrong with it.

For a list of links to maple problems from Oregon State University...

Here is a link that might be useful: Index, An Online Guide to Plant Disease Control, Oregon State University

It is impossible to determine exactly what might be the problem without photos. 'Sango Kaku' is a bit more prone to some Japanese maple problems than many other selections but just based on your description, it is hard to offer any constructive advice. And yes, you CAN make the problem worse by overwatering.......depending on exactly what the specific reason for the dieback and drying leaves may be.

Post photos either here or on the Maples forum for better advice :-)

Thank you for your response. I agree photos would help but I don't know how to do that. The link on diseases was helpful. However, the closest is verticillium wilt but I don't see the vascular discoloration. I'm still thinking it might be lack of water. I guess I'll just have to see how it plays out.

While common with VW, vascular discoloration is not always present. And verticilium tends to present later in the season as well and often with very rapidily developing symptoms.

The easiest way to post photos is to sign up with a free photohosting website (like Photobucket or Picasso), down load you photos then link them here.

Here is a link that might be useful: how to post photos on GardenWeb

I think the problem is lack of water to the roots. A small rootball in the company of Doug Firs tells me this.
Tom, is the tree planted in lean, fast draining soil?
How close to the firs? If planted in the sun under large firs, it won't be happy.

Any time there is less than one inch per week of rain a monsoon climate tree like a Japanese maple may need to be watered. Even without large conifers affecting the planting site. And what these often do is deflect rainfall away from their root crowns with their foliage, so that is is dry near their bases even when there has been quite a bit of rain.

Typically in this area waiting until July to start watering is too late for many kinds of plants on many local sites. It depends on what kinds of plants are being grown, how well established they are and what the light exposure and soil texture are.

Across the street from me a bigleaf hydrangea grew for a long time on an airless, cement-like glacial till soil in full sun, without consistent watering. I never saw it wilt.

If I walk down and look it may still be there. Shaded specimens under trees on a more normal ~garden soil up here on our place have started flagging.

My neighbors planted Douglas firs as a hedge in 1950 and never sheared them so I have 60' tall Doug firs along the back of my property and I have to garden under them due to lack of anywhere else in back yard. (Small house, small lot) Not only do the roots extend twenty feet into my yard under my bed and lawn but the branches extend that far too. The branches are very thick and block rain from my bed. We've pruned them off twenty feet up but it doesn't matter. The bed underneath is dry even when it is raining.

I have struggled for sixteen years now because the roots suck so much moisture. I built the beds up and added yards of compost and the feeder roots just grew up into my good soil.

Soaker hoses have not worked very well, I have to hand water. Many perennials have withered away especially hostas, huecheras, tiarellas and some shrubs have died (multiple azaleas evergreen and deciduous) but some plants do survive. I have had good luck with Pieris and the native bleeding hearts. I'm about to give up on Clematis which is upsetting because I have forty feet of cedar fence to use as vertical space for them.