This could be caused by any number of reasons.
Check the Warp tool brush size. On the right side of the Liquify dialog box, under Properties, make sure that Pressure is set to 100%.
Make sure that you are not working on a locked layer. If the layer is not locked, make sure that you are working on the corret layer.
It could even be an issue with your graphic processor -- Under Preferences > Performance, try turning Use Graphics Processor on or off. And, make sure that your graphics processor is up to date.
... View more