I just tried the shot you suggested (shooting from 7 diamonds away) and it's actually hard to bridge from there. That said, on my 9' diamond I can get around 18-20 diamonds of draw, with a slightly elevated cue due to the rail. My cloth is maybe 4 years old, but the balls are pretty clean. I shoot with a Cuetec Cynergy 11.8 with whatever tip comes from the factory.
If I was going to guess, I would suggest that your intended tip position is different than your actual contact position. You can verify this by taking your phone and getting some video from the side when you shoot a draw shot. This will confirm/deny your contact point vs intended point, and perhaps illustrate how you can aim lower without getting to your miscue point.
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