Sorry I forgot to mention. My iPhone 5s is running iOS 12.5.2.
I get more luck when sending via the Linux Jami client.
As a test Android vs iPhone I let both phones go into standby at the home screen for about 30 minutes and check their presence from the Linux client. For me iOS has a habit of showing Offline where Android is always showing available. Just a small thing moreover it would explain why i do not get notifications of new messages on my iOS device until I wake it up and open Jami - then I get a stream of “queued messages”.
I’m still staying with Jami despite some of my converts have gone back to whatsapp due to the “quirks” of Jami.
The highlight from my converts of past and the have left Jami is file transfer. As a average user maybe adopt a “Wait and retry” rather than a “Fail and give-up” method for File Transfer. I have succeeded in initialising a transfer after 2 or 3 retries. It seems once the file transfer session has started it will succeed.