Returns being declined five times

I have logged a return for my son's device which I bought in May and in June it started to give me issues where as it switched off and when trying to put it back on it showed me the witouche screen without going on. Which I then submitted it for return. I have then logged it now for the 5th time and it keeps being declined because apparently there is nothing wrong with it when they test it as it worms them stops then gives various errors when I have it which for me is a huge inconvenience because now I need to share my phone again with my son which is ridiculous because I bought him one to avoid having to share.