Did Cheung Chui San's martial arts improve during his 10 years on Ice Fire Island?

Recently, I learned that Golden Haired Lion King Tse Tsun's martial arts improved greatly during his twenty years living on Ice Fire Island. Part of this, apparently, was the result of the temperature extremes of the island itself. Cheung Chui San and Yan So So spent ten years on Ice Fire Island, but when they returned to China, Cheung Chui San's martial arts were said to have fallen behind those of his Mo Dong Sect martial brothers (due to having been away from wulin for ten years and out of practice). Why did Tse Tsun improve but not Cheung Chui San?

It is not that ZCS deteriorate, it is that he did not improve as much as what he was when he's at Wudang.

While his apprentice brothers continue new techniques developments, ZCS have to contend with catching fish etc for survival. He continues to train but probably not to the degree that he was back in Wudang.

Han Solo

he probably improved his kungfu but only what he had learned previous to arrivng at the island. his brothers had learned new kungfu and he hadn't so they improved faster then him.

Zhang Cuishan's alleged lack of improvement was simply his own perception of his martial arts skills.

In Chapter 9 (2nd ed), he said to Yin Susu: "Being away for the past ten years has left me even further behind (Yu Lianzhou). Sigh, after being deprived of my master's instruction for a decade, I may as well retire to the end of the line."

Later on, after fighting Quan Jiannan from the Green Dragon Clan of Korea, he said dejectedly to Yin Liting: "My strength alone was just insufficient to protect (Yu Lianzhou). After being away from our master for ten years, my skills have fallen far behind those of my brothers."
Yin Liting replied with a laugh: "What are you talking about? Fifth Brother, you could have easily dealt with the Three Rivers Gang without me. But you chose to take care of Second Brother first, and allow Fifth Sister-in-Law to suffer some fright."

So:
1. Only Zhang Cuishan himself felt that his martial arts skills did not improve.
2. His brothers in Wudang did not say anything negative about his skills, regardless of whether they were being nice or honest.
3. The people whom Zhang Cuishan fought with during the period between his return to China and his death were generally not his match. The only one who could have given him trouble was the Xuanming Elder who abducted Zhang Wuji, but he did not have the opportunity to tangle with him.