One reason why this behavior could happen is that the uicontrol is a child of a uipanel object, but is placed outside the scope of that uipanel. (For example, placing a button on a uipanel, and then resizing the panel so that the button lays outside of it will reproduce the issue).
To resolve this issue, there are two options:
1 - Resize the uipanel so that the uicontrol reappears
2 - From the Object Browser panel, select the uicontrol (double-click on its name to open the property browser) and change its position property so that its location lays inside the uipanel.
