I think you stopped before finishing. Mine looked like that at first. But then I went around and pulled a staple here and there and stretched it some more. You particularly have to stretch and restretch it where it wraps around the tank, to get all the wrinkles out of the "valley." I found I was using the stapler head, itself, to do the final stretching as I stapled. Hopefully, you still have enough excess material around the edges to get a decent grip for restapling. I started at the back, initially. I don't know if it makes a difference, but I also oriented the material with the lines on the backing running side-to-side, so there was more stretch front-to-back... I think.
Here's a pic. (I was actually taking a pic of my R/R, so a bit of the front is clipped, but it looks pretty much perfect, everywhere). It's hosted at Photobucket. You can click on it with the magifying glass to get a close-up of the grain.
http://i18.photobucket.com/albums/b1...recover003.jpg
18.00 for the material (15.00 plus shipping)... it's marine upholstery grade vinyl, and I have enough left over to do it, again. This is actually much thicker and supple than the original cover... although the original cover was worn paper thin at this point!
3.00 for 6mm staples (short ones)
22.00 Stanley electric stapler gun (already had)
BTW, your old seat cover still looks NICE. Mine was 90% duct tape before I was brave enough to try this. :)