planet will not form

When playing SE5 in simultaneous Movement mode i found a small problem i have fitted my ship with all that is needed to build a planet and i moved to an asteroid belt and use the option to creat a planet the next trun the new fash comes up i have created a planet but when i go there nothing is there at all and the astreroids have gone but no planet this only happends to me in simulataneous movement does anyone know how i can fix this problem .. thanks for any help