Have you checked that any isolation valves are fully opened? Seems obvious but an easy oversight.
Also has a high pressure cone been fitted instead of a low pressure one?
Otherwise there could be a blockage with grit, etc., internally to the flush valve.
Stephen
December 2009
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.