Grout between tile and coping falling out?

[Shredhead]

I have a poured concrete deck and coping in northern IL. The 3/8" thick grout between the top of the tile and the coping is falling out after just two years. The grout between the tile is only 1/8" and is just fine.
It seems like the expansion and contraction during the winter cracks the thicker grout?

Any suggestions on alternate materials?