Hello Don!
I don't quite get it - what tabs do you mean? Is the part that will not fit the blue one, on heavier cardboard?
In paper models it's a common problem that a bulkhead doesn't fit - it's usually too large, because the designer often forgets to the thickness of the paper into account. Please remember, that you always have the option to sand the bulkhead down before installing it. I hope it help, or if I don't understand the problem right, please post more photos and I hope we can figure it out together!
Good luck with your build and have a nice day
Paweł