My customers log in to their accounts but when they open subscriptions, it says that they aren't logged in

If your customers log in to their account in your shop, but when they go to the /a/subscriptions URL it says that they aren't logged in, then you are probably using the new Shopify's customer accounts feature, where customer log in to their account with email and not with a password. 

We have noticed that sometimes, these new customer accounts experience an issue where the actual session isn't passed to our app. We have reported this to Shopify and they are working on the fix for this issue. 

There are two options to bypass this issue:

a.) You can start using our new customer portal, which is directly integrated into the Shopify's new customer accounts. You can read on how to enable it in your shop here: https://www.sealsubscriptions.com/articles/frequently-asked-questions/how-can-i-add-the-link-to-the-new-customer-portal-extension-ek

OR

b.) You can simply turn on the “Legacy customer accounts” in Shopify, as these accounts don't have this issue. 
To turn on the Legacy Customer Accounts, go into Shopify admin > Settings > Customer accounts and turn on the legacy customer accounts.