Do you have Stacked Marks turned off for this worksheet? You can check by going to the Analysis menu > Stack Marks, then see if it the option is toggled to On or Off. If it is set to Off you may just need to switch which marks are in front. You can right-click on one of the axes and select either Move Marks to Front or Move Marks to Back depending on which axis you have clicked on.
If that doesn't fix it would it be possible to share a copy of the twbx?