problems with zymography gels - (Feb/20/2008 )

Hi,
I'm using the novex gels from invitrogen: 10% zymography gels (10% trisglycin, 0.1% gelatin). I have problems with the samples I load in the edges of the gel, they always come out fainter that the samples I run in the middle, ddo you have any ideas why this happens?

more total surface area for diffusion of the proteins (including diffusion out of the gel).