Simplenews subscriber list disappears/is cleared now that I have a second newsletter

I have subscribed users, or they have subscribed themselves and already used Simplenews successfully to send the newsletter.

Now, however, I have created a second newsletter. I have subscribed a list through "import subscribers", but after i have sent a test newsletter to the test address I noticed that there are NO subscribers assigned any more. Strange.

I noticed this problem "went away" when I deleted this newsletter altogether and created a new one. A drupal developer suggested that the names of the newsletters were too similar and a mix up was occurring?

More worrying, when I added a third newsletter I lost all the subscriber detail from my original newsletter. This sounds awful, but interestingly all the subscribers are still in the database. What is it about the addition of these new newsletters that could be so drastically messing up the display and my ability to manage newsletters and subscriber lists?

Very very keen to get this resolved so any thoughts are welcome.