Customers shouldn't normally not receive the download link after purchasing your plugin. However, there are a number possible explanations for this, including the following:

There's an issue in the plugin code. This is the most likely case, especially if the problem has just recently started.There is an issue with the plugin's store integration. This is also possible, especially if it's your first retail platform.There is a problem with the user's account. This is still possible, even though it is less likely.  Chilis Survey