Complex sentence: There used to be a lot of students who joined the club, but recently the number of students decreased due to Yau Dak's lost of interest in swimming and hence putting less time and effort into the Angels Swimming Club.
Split into simpler sentences: There used to be a lot of students who join the club, but recently the number of students declined due to Yau Dak's lost of interest in swimming. Therefore putting less time and effort into the Angels Swimming Club.