how can i fix my cistern?

the cistern in question is a Derwent macde,Pacific dual flush valve which i have just replced because the old one was weeping into the pan.the new one that just twists and locks into place has the odd weeping moment now and again but also does not want to flush if the cistern has been left overnight.if you take the valve out the next day then replace it to fill up it will flush ok untill its left for a period of time.any surggestions please,thanks very much