X11 forwarding not working for same id, signed by different people

2 users both sign onto the linux box with the same id. X11 forwarding works for one and not the other We tried both putty and mobaxterm. We think we verified all the options between the 2 users. All the settings in putty and mobaxterm look the same between the 2 users. But x11 forwarding works for one and not the other.