Lam Chiu Ying disappeared fairly early from Central Divinity Wong Chung Yeung's life. When they last spoke to each other, Wong Chung Yeung had not even founded the Cheun Jen Sect yet.
It's not really known how long after their final encounter that Lam Chiu Ying died, but I would reckon to say it wasn't too long afterwards: most likely before the First Mt. Hua Sword Tournament because, after all, Wong Chung Yeung himself didn't live for long after that struggle. What that means, however, is that Wong Chung Yeung probably developed the bulk of his martial arts after breaking ties with Lam Chiu Ying.
If that's the case, why would the Cheun Jen Sect's members have so much trouble with the Ancient Tomb Sect's martial arts? Lam Chiu Ying was very familiar with Wong Chung Yeung's pre-Cheun Jen Sect martial arts, but did she ever see the martial arts he developed in the subsequent years? Some of the Cheun Jen Sect's best martial arts, including the 7 Stars Northern Dipper Formation, weren't developed until fairly late in Wong Chung Yeung's life...long after he and Lam Chiu Ying had ceased contact. Just because Lam Chiu Ying had Wong Chung Yeung's number in the martial arts he had during his youth doesn't mean she should have the upper hand on all his subsequent martial arts he developed in later years.