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Depending of the slide out system, if electric, the brake may be failing. If hydraulic, the valve on the manifold may be partially manually open. make sure all the valves are manually closed. If they are then the seals in the valve are failing. One other hydraulic system possibility is air in the lines. This would only occur if you have had to add fluid to the system.
Give us more information and background and we may be able to identify the actual problem. Thanks.