This is very common. The Blue Seal pellets are probablty sweeter and richer – and less healthy! – than the Oxbow pellets. That said they are probably ok for a growing bun under 6 months, because what show-breeders want is for their buns to grow and plump up, and that’s what we want with our little ones too. I looked up the product details on Blue Seal Show Hutch deluxe, and it doesn’t sound too bad. I compared it to Oxbow essentials Young rabbit food:

And here’s the Blue Seal:

The main difference as I see it is that the oxbow has more fiber. Fiber is super-important for bunnies. Hay has lots of fiber, so if your bun eats her hay like a good girl, I think she’äll be ok if you keep mixing the Blue Seal with the Oxbow for another month or so, since she is still growing.
As long as she’s eating hay and vegs, it’s ok if she doesn’t finish her pellets until in the middle of night when she has nothing left but the Oxboy and hay to eat. That’s how my buns did it when I switched to Oxbow: they didn’t devour their Oxbow but saved it until last. You should never try and starve a bun to get her to eat “when she gets hungry” like you can with a picky dog, so it’s important she eats hay as well and greens.