I have been through similar times with Trixie. Take a look at her SS from this year, starting at the end of March and through April. At that time, I changed her diet, and it changed her insulin needs. I had to take her from 2.50 to 5.00 (systematically following the protocol) before I hit a breakthrough dose. During that time, I was scratching my head - a lot. I'd increase her dose, and the increase seemed to have no effect what so ever. But, suddenly, I hit a dose that worked, and now we're heading back down the ladder.
I'm sure others may have more scientific insight, but for some of us, this may be one of the anomalies of FD. With that said, you may also want to double-check to see if any sort of infection is brewing, too, if you can.
You're doing a great job with those increases....you will eventually hit a working dose.
Have a great evening, guys!