Youngsai, I have HC in Eco Complete and was having the same problem. In a nano tank and (too) aggressive gravel vacuuming it was getting disturbed and coming un-attached. My Eco Complete also sort of settles out the fine grains, which I understand it's supposed to, but then the top layer was kinda big.
I finally took a small amount of well washed sand and sort of sprinkled spoonfuls of stand right on the hc patches, figuring it would help root it and it would grow through and out into the light, helping to anchor it and encourage new growth. So far so good. I also am much more gentle with the gravel vacuuming now.