I have an excellent website for you to read about the ears. I have also read that while they are teething it is common for their ears to be floppy.
I do not remember ever having trouble with my older yorkie (she is 11) but my 5 month old will have days when they are floppy and days they stand up. The larger the ear, the harder it is to stand up. I have taped my little ones ears, they stood up for a while but are starting to flop. So I am not an expert. Take a look at this site, she has info on lots of topics:
Jon-An's Yorkshire Terriers (How to train Yorkies ears to stand)
Good Luck!
