I'm going to assume you are running ZB and not SE firmware.
A few things could cause this problem. By default the LTH will join the first pan id that it sees. If you have other coordinators in the area it might help to disable them until you can get the LTH to join the xstick.
In order for the sensors to join they must match firmware, pan id, encryption, and have compatible sleep settings. The coordinator must also allow joining (see NJ value). ZS values, Zigbee Stack, must also match.