Such a problem usually occurs when the client-side object of the RadSlider is not created. Since the control is rendered on the client by default, it will not appear if its client-side functionality is broken.
Please check if you have set a property for client-side event of the slider
without declaring the handler or if you have specified the handler correctly in the event property (there are examples in the help articles that are referenced in the article linked above).
All the best,
the Telerik team
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