Can't delete specific conversation on Android Messages app.

[Android Central Question]

Hi,

I have been trying to delete a particularly long text conversation on my Samsung S5 Active, but have been having trouble. Lately when sending text messages ONLY to this specific person, sometimes messages won't send or I won't receive messages and it also takes a really long time to send messages (i.e. it will say "sending..." underneath a message for up to 30 seconds). However, messages in other conversations with other people were working fine!

I thought these problems were because I hadn't deleted this conversation's history in a while so I tried to delete it. It deleted OK, but when I tried to open up a new, blank conversation with this person, the app wouldn't allow me to! So I restarted my S5 and the whole conversation was back in my messages! All the texts I thought I had deleted from over two years ago will not go away. I believe this is causing problems with sending and receiving messages to this person.

Anyone had a similar issue? Any idea what could be causing this or how to fix it? Thanks!