There are so many factors to consider and all of the suggestions submitted are really great. I recently tried something that seems to help with that same problem. I had been tying my reeds so that the top thread lines up even with the end of the tube. I experimented with tying the reed going just past the tube and others just before the end of the tube. The ones that were tied just BEFORE the end of the tube kept their opening much better. They seemed to be a healthier reed as far as the opening goes.
Also, Cheryl Wefler in her "Principles & Techniques for Oboe Reed Adjustment" offered an idea. She said that some oboists after shaping their cane will bevel the ends of the cane (the part that is covered by the thread) so that the thread will wind smoothly and not show a bump. I don't know if you do this, Laura but Cheryl says in her book that beveling the cane like that weakens the blank. She also recommends removing the ears before tying which could affect your opening.
BTW, I keep a reed journal with specific information on what I do on each reed. I log in info such as tie length, cane, diameter, shaper, staple, etc. Since I have a lot of different colors of thread, I then identify each reed by its thread color. Sometimes a reed will not blossom for me right away so when I find a reed I particularly like, I'll check the journal and notice what I did differently on it.
Hope you find a solution to your question, Laura!
Oboe, English horn: Amarillo Symphony, Chamber Music Amarillo, Amarillo Opera, Shepherd's Trio
Composition & Arranging Website: http://janetlanier.com
"Ever notice how orchestra pits shrink through the run of a show?"