My experia xp has lost the light effects?

My sony experia xp seems to have lost its light effects/notification lights. Oddly enough, only for unread texts. I have set the light effects again but to no avail. All other effects work correctly. Maybe, and perhaps you can answer this, the light effect only works after so long leaving the phone. It is annoying as I cannot now tell at a glance if I have any new texts. Any help, greatly appreciated. Thanks.