The most likely reason is that the cake mixture is too soft. Try ading a little more flour to the mixture. Also try coating the raisins in flour before adding to the cake mixture. The same applies to other fruit such as glace cherries.
Dear florence ,thankyou so much for caring enough to answer my plea for help!
Every year at the jewish new year i make this cake -it's the best honey cake i've ever had,but there was the (sinking raisens)the batter is very thin so i'm wondering how much extra flour i should add,along with the floouring of the raisens.I'll let you knnow hoow it turned oout! Thanks,again,joan davis