Waverunner seat reupholstering question...

I am trying to reupholster my seats on a 98 Yamaha XL760. I got the new seat covers and a good electric staple gun, but when i try to put the staples in they just bend to the side. I am using Arrow T50 1/4" Monel Rustproof staples. Do I have to use a thicker lower gauge staple? I see that they make a 20 gauge 1/4" stainless steel staples, do I need to use those? Thanks for your help...