The location of the inputted text is set here in the button definition, but im not 100% sure if that's the issue.
Do you have any other code in your project modifying the styling of text buttons? They look like they might be centered, which is affecting the positioning. Try adding 'xalign 0.0' to the end of the line with all the button's details, that should set it to left align which should make it easier to fit it into place.
If this helps let me know, otherwise I can try a few more things to figure it out.