Recessed Lights...Can get the 'pins' to stick

Just putting in recessed lights. There is a layer of drywall and then plaster so fairly thick. I tried bend the pins but that won't snap in. I can't get the black push 'pins' to hold up the can. There isn't any strapping to screw to. Could I screw use a machine screw to hold it to the plaster or would that hold?

Any adivice?