My experience with the 4x5 version of this tank is that certain film/developer combos cause issues while other don't. What I finally settled on is a...
I agree with Philip it has to be something going wrong with exposure (or the off-chance something is very wrong with the box of Delta 100 you have)....
I have not worked with these developers before, but one thought is that you may need to dilute the developer more. Development times under 10 minutes...