Try placing the coverslips in a p24 or p48 well plat and filling the well with enough gelatin to cover the whole coverslip. Are you doing it this way?

Yes, and then aspirate off extra

hI Hilaire, It could be by drying. You could place the coveslips into an incubator with a water supply or add 0.01% glycerol to your reagent  to prevent drying at RT.

Also if the gelatin is not heated to +75C and be homogeneous it tends to separate out quickly.