Hi,
There are so many reasons and without knowing all the details it is hard to pinpoint exactly which is the issue for your cakes.
1 its summer in England, is your frosting in the layers softening causing the tiers to move with the weight of the above tier
2 what supports are you using? Are they strong enough?
3 maybe the cake boards are not sturdy enough and the supports are going through.
Cheers
Kylie
