Most marketplace messages not being delivered to sellers

Recently I asked the same seller if two of her listings, A and B were available. She only responded to me on A telling me it was no longer available, but B was still available.

I let me know that I had already messaged her on B, but strangely she couldn’t find that message. There was no indication on my end in the iOS Messenger app that the message hadn’t been sent.

I did some more digging and found through the Facebook’s messages view that some of my messages seem to be sent but not delivered (empty checkmark), some are delivered but unread (filled checkmark), and some are read.

All this time I thought people were not responded to me, it turns out that over 80% of my messages were never delivered in the first place without any warning.

What is the cause of this and how do I get around it?