Bridgeport 720X Heidenhain 2500 will not reference X axis!!

Hi all,
When I start my 720x machine, it finds the reference points for Z and Y axis, but not X.
When the start button is pressed, the axis moves to the first limit switch, as it should do, then just stops instead of changing direction and finding the reference point. I have recently had a problem with a Gross Positioning Error, which I have now fixed, but I now have this problem. The encoder is working properly, and I have tried moving it around the spindle, but it makes no difference.
Has anyone had this happen, and if so could you give me a fix please!
Thanks,
Andy