When we fix the back cover at our shop we always spread just a little Loctite in every corner, sides and top + bottom. Make sure the battery doesn't have bumps sticking up from the back cover removal, try to "massage" them in so it gets flat again. The original back also has white sticky tape over the battery, if you can't get the exact tape don't bother putting other adhesive except fluid glue which doesn't harm the battery on it. Remove all the white sticky tape unless perfectly flat. Even the slightest push from below the back cover will loosen the corners eventually with only tape.