The gradient tool in Inkscape is a powerful feature that allows artists to add and edit linear and radial colour gradients on-canvas. Gradients in vector graphics provide the ability to provide the illusion of depth and light to otherwise flat images. When combined with the blur filter, gradients can be used to create photo-realistic vector images.
However, having gradients behave strangely when moving or resizing an object is a common issue encountered by Inkscape users.
You have an object on the page that contains gradients. In the following example, the object is a group of shapes depicting a shiny hammer.
But when you move the object around on the canvas, the gradients change. Colors darken or lighten, gradients disappear or become a solid color again.
In most cases, when this behaviour is encountered, the object is moved but the gradients are not . If you were to un-group the objects making up the hammer, and choose the Gradient Tool, graphic representations of the gradients in the objects would become visible:
Compare to how the gradients look after the object is moved:
The gradient handles have remained in the location that they were before the move. Note, also that if the object is resized (scaled), the gradient handles would not have resized either.
There is a Button in the Tools Control Bar for the Select Tool that lets you toggle if inkscape transforms gradients when an object is transformed.
When this button is toggled on (pressed), inkscape gradients should start behaving more as you expect.