Fullness is a calculation resulting on the number of eggs, incubation progress and size of eggs, determined by each critter. It is normal to not go to 100% and will stop rising when progress reaches 100% :).
Also, on version 1.30, Parasite Worms always go only up to 28% fullness. If you want to rise it with normal incubations, you have to get rid of the worms first, because they eat other eggs to grow.