Gas gauge not working

I just swapped the guages on my '87 XJ. I had the idiot lights with the big gas guage and the speedo. I put in the guages with the gas, speedo, oil, battery, and temp guages. I know i need to replace the sensors for the oil and temp guage to work but my gas guage isn't working. When I initially installed it and started the Jeep the gauge went to full and passed it, going to about 3 o'clock and hasn't moved since. What could be the problem?

Thanks,
Adam

check these out...

http://www.naxja.org/forum/showthread.php?t=98672&highlight=gas+guage
http://www.naxja.org/forum/showthread.php?t=98081&highlight=gas+guage