If the toilets work properly but do not rinse when flushed, it could be the rinse valve filter. This filter can be cleaned if you remove the valve. Get access to the rinse valve by removing the lavatory covers and remove the valve. When you remove the valve, only take the top two clamps off. If you loosen the bottom clamp slightly you can pull the valve out (the bottom clamp is quite difficult to put back on). When the valve is out, undo the bottom part (where the water enters the valve) and you will find a small flat filter which can be cleaned or replaced. When you re-install the valve, put a small amount of vaseline around the bottom of the valve so that the clamp can pass over it before re-tightening it.
Thanks to a welshman-abroad.