inconsistent results with email froma workflow

Hi,

I’m a newbie teaching myself SharePoint skills and I have run into one of those irritating problems where everything appears to be correct and yet it does not work.

I’m developing a simple form that will tell people when a parcel is waiting for them in a mail room. I am doing this on a site hosted on sharepoint.com

Using Designer 2013 I created a list containing all the relevant data including a field which holds the recipients email address. I then added a workflow to send an email to the aforementioned email address when a new record is added to my list. I had to chase around for a while before realising it will only work if I specify that I am using an SP2010 workflow, however once I corrected this mistake it worked.

The next day I tried the same method again but it does not work. The workflow reports that it has Completed but no message arrives in my test mailbox. The original list & workflow still work – they send an email message to the target address I use to test both lists.
I’ve checked the things multiple things in Designer but I can’t see what is different about them. What am I missing?

Thanks in advance!

Pete