That’s very odd then that they won’t stick, even with poly-lysine. Maybe the Cellvis plate has something on it that is making the cells unhappy? Do the cells adhere when you put them in a 6 well polystyrene plate with a #1.5 coverslip inside. It wasn’t clear if you used the coverlip inside a cellvis plate or just a normal polystyrene plate.
Alternatively, you could try and use a polymer coverslip like Ibidi. It has properties similar to a #1.5 coverslip. Just be careful what oil you use as some of them damage the polymer.
Also there are the Labtek and Matek glass options that you could try.