Develop the art of listening. Get a stethoscope before you stick and listen to the sound of the blood flowing through the graft. I listen across the access from left to right to hear the strongest sound of the bruit. You do not have to see it or feel it to be able to cannulate it. Femoral grafts tend to have higher pressures. Grafts can be so deep that the angle of insertion may need to be increased, so you feel the needle's hub is standing straight up coming from the graft. That's ok, as long as you have a good flow. I use a "pillow" 2x2's (folded x 4) to prop the hub so it is more stable before taping across and then butterfly per your unit's policy. Diablo's point of tightening the end cap before you cannulate is invaluable, it is a second nature thing I do now without even realizing.
Sadly, nurses tend to have less cannulation skill and expertise than techs, as they are doing the many catheters and other duties during changeover when you need someone. Sometimes, backing up a hair and reangling the bevel inside the graft by moving the hub that you are holding is the trick. It takes time and practice to develop this skill. Patience is the key. Take a deep breath and don't give up. You're spunky....you can do it!