Put the summary of the bug you're having here

What I expected to happen was...:
Using the /fill [x1 y1 z1] [x2 y2 z2] stone replace air, I expected it to fill all available air blocks in an area. (For example, a cave system.) I ensured that the space was small enough that I did not return a "too many blocks" error.

What actually happened was...:
It will fill only a few blocks, and/or otherwise will not fill in all available air blocks with stone.

Steps to Reproduce:
1. Find a cave system
2. Enter test /fill [coordinates 1] [coordinates 2] stone replace air (coordinates will be small enough that the number of blocks is not an issue)
3. Receive error "No Blocks Filled" or only a few random blocks will be filled.