To add to what Evil Fefe said, there are two characters whose X-Ray Kombos might be hard to pull off--Johnny Cage and Cyber Sub-Zero. For Cage's X-Ray, wait until your opponent attacks, then pull it off, as his only works as a counter attack. For CSZ's X-Ray, you have to jump into the air before pressing LT+RT.
Hopefully, this will help those who are having trouble with either character's X-Ray Kombo.