Some simply don't stand up. I have seen many people float ideas such as shaving the hair off, etc. Not sure those things work.
Many have floppy ears up until 9 mos or even longer. It often is related to the teething process. I had one with one ear up and one ear down....suddenly at 10 mos, her ear stood up.
They are adorable pups with erect OR floppy ears!
