Unable to dig or drop dirt on certain tiles.

[Galeck]

As the title says I seem to be having issues with dropping dirt or using the dig command on certain tiles.
I've claimed an area and am now flattening the terain around it so I can expand and build more easily.
However, when I use the dig command my guy simply refuses to do anything, for instance when trying to dig out the area marked as red here: (I'm using the Ender client) http://i.imgur.com/MS1tG.jpg?1

The rest of the terrain around it is also giving me trouble, cause I figured if I can't dig it out, I'll raise the rest, but it also refuses to drop dirt on the rest of the area...

I'm quite confused as to what is going wrong here, I hope someone can help me out.

[Naithin]

It seems that you can't dig or place dirt onto sand, I came across this problem earlier with a small corner of a planned second claim area, ended up having to move along a bit as a result.

[Galeck]

So basically I'm screwed and I should try to relocate?

Well... there goes my dream of a beachhouse