This could be two, likely, but separate issues. The first is that it could be over-watered, and thus contracted an infection. You can attempt to correct this by adding a little dolomitic lime and wettable sulfur.
Luckily, this will also correct the second likely scenario- A magnesium deficiency. The dolomitic lime will act as a pH buffer, while providing calcium and magnesium for the plant.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/vines/snail-vine/growing-snail-vine.htm