Probably atmospheric temperature is high there. Do the job in cold room.
Make sure filter each solution (should be 2x solution) through Whatman or spin down hard to remove any gelling cores from the sol. Then mix to medium to make a final conc.
in addition, the concentration of the samples might be too high, have you considered a gradient % of collagen and gelatin? also the type of collagen would also effect on its physical properties.