long lag in recieving mail notifications- recent problem.

there is a long lag of up to an hour in recieving notifications in my inbox from forums & other sites, often up to an hour after they appear on the sites themselves. indeed, as often as not i see the messages on the sites themselves long before they appear as notifications in my inbox, thus defeating the object of the notification. but this has only started happening in the past coulple of months- they used to appear within minutes of the sites sending them.