Depending on your bank, the connection is valid for 90 or 180 days.
If you no longer see transactions coming in, you need to take the following steps:

1. Go to 'Settings' > 'Configuration' > 'Bank'.
2. Click on disconnect for the desired bank account (do not delete!).
3. Now click on connect to link the account again.

The missing transactions will now be retrieved, no data is lost.