Emax Mini more battery drain problems

wryly

Now I wonder if anyone can explain a new power drain problem I had.
Prior to travelling, I charged my phone and it was charged by 11pm. I didn't attempt to use it until the following day. When it came to checking the email for the reference number for my train tickets, and it was completely dead. If I hadn't been carrying a spare phone, my journey wouldn't have happened. Not good.
This time, I can't put it down to texts failing to send, as there hadn't been any cued. I'm tempted to return it to the seller fopr replacement, but the fact that it worked adequately since (battery life is still considerably poorer than when new, just over a month ago).
Are there any known issues? The intermitancy of these battery drains makes me think it's something to do with software, rather than a harware fault, as I'd expect it to be consistently draining unnaturally fast, but it isn't performing as well as it should, or did.
Any advice?