Emails not being sent to subscribers

I was not the person who created our site, but I do my best to maintain it. So, pardon, in advance if I don’t understand something.

Our site users subscribe2 to send emails anytime we send a post. Normally it works fine, but on various ocassions, it doenst consistently send, or only sends to part of our list. With no rhyme or reason.

Please help.

This is a site for a Boy Scout Troop, Thanks in advance.

https://wordpress.org/plugins/subscribe2/