GeoServer – Why is my WMS map not rendering when previewed in OpenLayers?
Sometimes when you use OpenLayers to preview a WMS in GeoServer the map window appears empty, even though when you click within the map attributes are returned for that location. This often happens when you are publishing complex layers such as OS MasterMap. In order to fix this problem you can simply increase the memory used by GeoServer to render your WMS. In GeoServer choose Services > WMS and then scroll to the Resource Consumption Limits category and increase the Max Rendering Memory (kb). I have doubled mine to now be 131072. You should now find that when previewing your WMS in OpenLayers the maps will render more quickly.