To avoid logging out and re-logging in every time you make a new purchase, please click on the + and tap on Slack to see the list of subscribed ebooks and Tap on the Sync button as indicated in the reference image below and ensure that there is no loss of internet connectivity when the sync takes place.

[Reference Screenshot]
This will ensure that your account gets uniformly synced and your purchases will be visible on the Bookshelf Tab ready to download.

If you are still facing issues, please uninstall and reinstall the app.