starch+iodine

hai ,
i have learnt that starch on addition to iodine gives blue colour ,
and when we heat the solution it looses colour & again reappears it's
blue colour . but when i was doing practically i found

1. when we heat the solution for long time , it looses it's colour &
doesn't reappear .
2. when we heat it just for fraction of seconds it looses colour &
it's able to reappear the lost blue colour .

WHAT are your comments ? would i have made any error ?