Halifax Building Insurance doesnt cover my water damaged ceiling, why?

my bedroom ceiling has been damaged by rain over a period of time (my tenants fail to report) they've only just reported it, and its come to a point where the ceiling is kneeling down and the walls near the window has lost its structure to the room...i showed it to someone today...who said its not just a case of plastering...ITS A BIG JOB!!probably over 5grand! I was expecting a couple of hundred quid job.... When i rang halifax home insurance, they said they dont cover it, as it happened over a period of time....i should have reported it on the first instance? what can i say to get my insurance to cover this....