One possibility is that you didn't start with a straight piece of cane, and/or it was twisted too. THROW THESE PIECES OUT! Don't think, "hey this cane feels nice, it's just a wee bit twisted." That's like saying, "Hey, God... can you please dispose of a few hours of my life, and make sure they're intensely frustrating, too!"
So, make sure the slip is straight, no twists, no curves. Yes, the longer the slip, the more difficult it'll be to find a straight piece. Does it have to be perfect? No cane is, but you need to try to get it as close to it as possible.
Next- your gouging, scraper usage, sanding cyl. or whatever you used to impose the inner curve, you weren't careful with the edges. If the edges (for example) have too much meat at the shoulder area, then it gets thin, and then again more meat near the top.... well there's a hole/your leak. You MUST be careful to make the edges as even as possible re: how much pith is left, or taken away.
Finally, where you position the staple can cause leaks/difficulty in the reed closing, the hardness/thickness of the cane, and many other variables (which I can't type now due to time). Check some of the things I mentioned, and let us know how you do.
Give a man a wooden reed and he'll play in the driest of weather,
Teach a man to make a wooden reed,
and the both of ye will go insane!