If there is a significant amount of wax stuck at the top of a lamp after fully cooling, then it is likely your wax has separated. That means the less dense wax separated from the more dense wax. This can be a difficult problem to fix, and if not done right it can ruin the lamp.
If the lamp flows, put a higher wattage bulb in the lamp to make the wax at the top to melt. The flow of the lamp should naturally cause the waxes to rejoin. If it does not rejoin you may need to gently swirl the lamp around to remix the waxes.
If the lamp does not flow, the lamp will have to be gently swirled, more so than if the lamp does flow. Be careful not to emulsify the wax and liquids.
If the wax at the top will not melt, use a hair dryer to melt it. Keep in mind that for the waxes to melt together they have to be close to the same temperature.
If none of these fix the issue then you can go for a more intrusive method. Drain 90% of the liquid into a CLEAN container. Then put the globe on its base and let the wax fully heat up. Gently shake and swirl the lamp to make sure the wax is mixed up, and then let it cool and refill it.
If all this fails, you can remove the liquid and boil the lava back together. Instructions here: http://oozinggoo.ning.com/page/chunky-wax
If none of this works it can be assumed that your lamp is dead.