I have 160 arborvitae trees lining my property that were planted by the previous owners about 2 to 2.5 feet apart. Ten have died. A tree company said they were planted too close together and I should remove every other tree. I'm not sure I want to do that!
You do not mention the species or cultivar of arborvitae that are planted. Mature height and width varies and it is helpful to know for proper spacing. Some grow to 50 feet high and 10-15 feet wide or more, while dwarf varieties such as Green Emerald could be spaced like your trees. When trees are planted too closely together there is competition for sunlight, moisture, and nutrients and the trees struggle. However, there are many other reasons for decline such as drought, poor soils, too much mulch, poor drainage, etc. See our online publication Common Abiotic Plant Problems for more ideas. An on-site evaluation of the trees and recommendations by a certified arborist http://www.treesaregood.org/ should be helpful to you.