glowworm ultracom 18sx pressure problem?

I have as above and the issue is the pressure keeps dropping over the space of about 2 weeks. When it does this it stops and flashes on the panel, then i re pressurise with the filler tap and the same happens all over again. I have had the glow worm engineers out three times and all have changed different parts and all haven't solved the issue.There isn't any leaks in the system that i can see anyway and there's no dripping from the bottom of the boiler. Any idea's....?