I also had a lot of trips to the US and was asked for my I-94 travel history. I was only able to get the latest I94 record, but when I was trying to get my travel history - the site kept telling me that there is no record for the traveler. Called US Border control and was told that this website for I94 is not a reliable source and only my passport can tell me my entry record to the US with stamps. So I had to find all of my flight tickets for 5 years eligibility period, print and mail them to CIC. It's been almost a month and I am still waiting to know if it satisfied their request.
Please try using your FIRST NAME AND MIDDLE NAME ( if you have it in the passport ) together in i94 website. I had the same issue and when i tried with First and Middle name together, i was able to get entire report.