Chosen Solution
I’ve done well over 30 systems now and I can tell you the issues come down to two! Bad cleaning! People don’t clean the surfaces properly and/or use the wrong solvents so a film is left. You want to make sure the surfaces are fully clean with none of the old adhesive left. I use a two solvent solution I use goof off to clean the old stuff off where needed and then I use reagent grade isopropyl alcohol 85% or better to pull off the goof off and any other oils using lint free lab wipes. Heat and pressure! You need to use a heat gun warm the case and glass surfaces to soften the adhesive so it makes better adhesion as well as apply pressure. You don’t need high heat, you want it warm to your touch and use a rubber roller to press the glass down along the edge (don’t over do the pressure as you can crack/chip the glass), This is what Apple uses! As you can see here:
