This is something that will plague those who try to use a magnetic phone mount on their trike, bike or car. I could not for the life of me get a metal strip to stick to the back of my iPhone 8+ without using a case, despite trying even the heaviest duty 3M tape. The answer was to apply clear 3M Primer 94 to the back of the phone first, allow it to dry for a minute and then apply the 3M metal strip. Hey presto, it now sticks! Something in the chemical compound of the material used to construct the backs of the newer iPhones reacts with the 3M adhesive and causes it to not adhere, hence the need for this primer. You can find small 10ml bottles on eBay for £5 shipped.