This behavior may have (at least) two reasons.
1) When importing an old slider into a new project, the old settings may not align with the new configuration. Please try and click the "save" button for the question to trigger the structural check for the question data.
2) If that does not help, you may have to set the proper position of "minimum" and "maximum" on the scale. To do so, please open the question, go to "Additonal Settings" -> "Slider" tab, click on the "position minimum" x-input (the line will highlight), and then click on the middle of the left end of the scale below. Do the same for the "position maximum" input, and eventually fine-tune the values.