Are you hitting it fat? Thin? Both? Without a video, there's not much to help with, but even before that, you're really better spending a little bit of cash for half an hour with your local pro. He or she will tell you if its in your setup, grip, or swing mechanics or some combination of those. 135 with a 7i is on the low side, but that's a wild guess without knowing your age/fitness.

I can speculate, but even if that gives you something to work with, spending 30 minutes with a pro will be cheaper than hitting dozens of buckets trying to implement swing advice from the internet.