Slimes not spawning in "slime chunks"

I use a third party software to detect where slime chunks are on a given seed and when i put in my seed and found the chunk and clear it away to make a farm no slimes would spawn. the software i use only has up to 1.7.4 but i have read that no updates have affected seed generation and slime chunks, could this be a bug with the pre release of 1.8?