Short answer: Keep filling away from the rim, remove trapped air, use an appropriate amount of filling, and seal clean, flexible wrapper edges firmly. Avoid overly wet filling.
Salted vegetables can release water, so squeeze or drain them when the recipe directs. Keep unused wrappers covered. For frozen dumplings, freeze them separately on a tray before bagging. Cook one test dumpling first; it reveals weak seals and helps confirm seasoning and cooking time.