Dahlia Disappointment

This is the second of my dahlias to go from looking very healthy one day to being gone the next. The first one lost all its leaves, and I assumed snails had got to it. I sprinkled some iron phosphate snail bait around to protect the remaining ones, but today a second dahlia has bitten the dust. This one has had the stem chewed through, but the leaves are untouched. Is this normal for snails or slugs? Or is something else the culprit?