Mainstream AC itself has evolved in a similar fashion over the years; traditional AC artists like Barbra Streisand, the Carpenters, Dionne Warwick, Barry Manilow, John Denver, and Olivia Newton-John found it harder to have major Top 40 hits as the 1980s wore on, and due to the influence of MTV, artists who were staples of the Contemporary Hit Radio format, such as Richard Marx, Michael Jackson, Bonnie Tyler, George Michael, Phil Collins, and Laura Branigan began crossing over to the AC charts with greater frequency. Along with Barbra Streisand, the Carpenters, Dionne Warwick, Barry Manilow and Olivia Newton-John, what adult contemporary staple found it more difficult to score chart hits in the 1980s?