An ear that is erect and now is floppy could be due to teething. What's happening in these pups is that their bodies are using their calcium for teeth development instead of developing ear structure. As a pup starts to get its adult teeth, it is, therefore, possible for erect ears to begin to sag and stay this way until they're done teething. In general, we expect all of the adult teeth to be in by the time a dog is 6 months, but all teeth may not be in up to the age of 7- 8 months.
You can ask your vet/breeder about whether taping is still an option in case you own a show dog that you are thinking of using for breeding/showing in the ring. If not, consider that a floppy ear adds character and individuality to your dog.