The ears can go up and down, or stay down, or stay up, or have one up and one down, or any combination of the above, and it's all normal. Some chis ears continue to do this throughout the entire first year, so don't be surprised if it continues beyond the 6 months timeframe you mentioned.
Jazz's ears did the up/down thing over a period of a couple weeks. They were both down completely when I first got her, and stayed that way till one just suddenly popped up one afternoon, when she was about 4 months. Then the other one followed suit a couple days later, then that first one flopped again....and it went on like that for a couple weeks. Then they were both up, and have stayed up since.