I have a longhair chihuahua. I bought him from a breeder when he was I think around 12 weeks old. His ears do not stand. I asked if they would and she said because of him being a longhair that his ears maybe floppy. My 5 other friends have longhairs and all of their ears stand. Was she lying? He is about 5 and a half months now. Will they mabye still have a chance to stand?
It could be that his ear cartilage is too heavy and they are not able to stand on their own or it could be that before you got him or during the time that you have had him, that his ears were damaged maybe playing around? Also it could be because he is teething. But if you really want his ears to stand you could try taping the ears to help train them to stand.
Chihuahuas are like potatoe chips, you can't just have one!