Now, when you say "het", do you mean both of their parents were dumbo or only one? Because if only one, you've got the dominant standard ear gene multiple times over the recessive dumbo gene. You would be lucky to get the one dumbo.
Try breeding your "het" dumbo to a pure dumbo, you should have much better luck.
It's like black rats... even if they had parents who were fancy colors, if you breed two black rats together, you will probably just get blacks. That's because black is a highly dominant gene. Dumbo is highly recessive. And that is why you didn't get many dumbos.
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~Sara~
1.2 ball pythons (Scorpio, Dayzee, and Satan)
1.0 Black-lined plated lizard (Lizzy Butt)
0.1 Burmese Python (Pixil)
0.0.1 Pacman Frog (Pudge)
5.2 Pet Rats (Emo, Skye, Siam, Lilac, Manson, Spade, and Zilk)
1.1 Dogs (Ozzie and Mandy)
0.1 Cat (Isis)
0.0.1 Synodontis Catfish (Big Spotty Fish)
0.2 Convict Cichlids
0.0.2 Crayfish