The String of Hearts plant can start dying due to not getting enough top lighting. It’s very common for plant keepers to keep the plan too high in the window and the plant doesn’t receive enough light from the top. It’s very important that the plant receives sunlight facing from above.  it’s a misunderstanding that trailing succulents like this don’t need lighting from above because the plant is hanging down.
The most common reason the String of Hearts plant starts dying is due to inadequate lighting. And you need to make sure that the Plante is getting light from the top down. This is one of the main problems when keeping trailing succulents. 
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