Those bull terriers are CUTE!
as far as i've seen, most dogs with prick ears are born with floppy ears. as the cartilage gets rigid and gets stronger, then the ears stand up. my ACD and my amstaff mix both have prick ears, they were floppy when they were puppies.
My ACDs ears didnt fully stand up on their own until she was around 6 months old. one would always flop over. and when it finally would stand up, the other would fall down again. it was too cute.
our vet said to also not play with their ears (bend them over, grab them) when they were puppies so as not to damage them.
Here's Chloe my Australian Cattledog at around 2 months:
Here she is at 4 years old:
And here's Honey, our Amstaff mix. her mom was an Amstaff, we dont know what Dad was. and this is also proof that even huge ears can stand upright! no taping or anything.