Your MyNook page should tell you when the next issue is released:
1) Click My NOOK at the top of this page (make sure you are signed in)
2) Click Magazines on the left side of the page
3) Any subscriptions you have should be listed and next to them should show the current issue and the next issue release date
Thanks for the answer. Unfortunately this does not address this issue. When I follow up the instructions, it does not show me any subscriptions whereas I have 3 in total (2 of them are being delivered without any problems). It does say that my account is not correctly linked to my credit card.
I assume this is because I sign in to Nook with my microsoft account (MSA); additionally the subsription fee was still deducted from my bank account end April. The thing is that the April issue is not even available in the Nook Library whereas it is available on the magazine website: http://www.jmouders.nl/Themas/JM-Tijdschrift-1.htm