Find one of he affected bulbs and cut it open. You may find a white grub inside. Narcissus root fly also attacks Snowdrops.
The other problem may well be poor drainage rather than leaves of the top. Snowdrops are a woodland species and are capable of coping with leaf mould on them. Check that the soil dries out in Summer.

Another possibility is that you live in an area of the country where there have been major freezes over the past two winters. This too can kill the bulbs at least according to Ian Christie who spoke to our rock garden club tonight. He also emphasised that snowdrops do not like sitting in water all year round.

Interesting MG. We have had major frosts here the last 2 winters and I am talking below -15c on a regular nightly basis without snow cover. Looking at my Snowdrop area we do not seem to have lost any of them to frost even the ones lying on the surface moled out are flowering ok. Our soil down there though is VERY well drained so possibly the frost is not as damaging if the soil is reasonably dry?
Definitely think that Snowdrops, whilst liking a moister soil than say Narcissus or Tulips, do hate standing water.

I was surprised as we've had frosts down to -15˚ too but again our soil is well drained. So perhaps it is a combination of poorly draining soil and heavy frost that does it. Will try to ask Ian when I see him on 19th.