Not Getting SMS Notifications If In Messaging On That Person

Hi All. Let me preface my question by saying I am absolutely reliant on SMS for communication while out and about because I've lost my speech to a form of Lou Gehrig's disease.

I believe this started with my recent upgrade to Android 5.0. My problem is that if I am messaging someone and they happen to reply when after I've shut my phone but before it turns itself off I get no notification, so don't realise they've replied. This has already caused me several problems.

If they or anybody else SMSs me after my phone turns itself off or if anybody sends me an SMS while I am in Messages on another person I receive notification, but not if the person I have Messages open on replies. To be honest, it used to occasionally annoy me that I'd receive noisy notification while I was in conversation with someone in Messages before the software upgrade, but now I realize that was the lesser of two evils in my particular situation.

I have been through the settings and if there is a way to address it I can't find it. The only way around it seems to be to sit and stare at my phone until it switches itself off - not always practical!

Thanks,

Gary