Answer by taking a quote from the following article:

Martha and the Vandellas (known from 1967 to 1972 as Martha Reeves and the Vandellas) were an American all-female vocal group formed in 1957. The group achieved fame in the 1960s with Motown. Selected Members of the group were inducted into the Rock and Roll Hall of Fame in 1995. Formed in 1957 by friends Annette Beard, Rosalind Ashford and Gloria Williams, the group eventually included Martha Reeves, who moved up in ranks as lead vocalist of the group after Williams' departure in 1962.

Reeves, out of the group temporarily due to illness, recovered and returned; Ashford was replaced by another former member of the Velvelettes, Sandra Tilley, and the group continued to release albums and singles into the early 1970s, although they could not reignite the fire that had made their records successful in the 1960s. Among their late 1960s hits was "I Can't Dance to That Music You're Playing", which featured singer Syreeta Wright singing the chorus, and peaked at number forty-two. Reeves reportedly hated singing the song sensing it "close to home". In 1970, the group issued Motown's first protest single, the controversial anti-war song, "I Should Be Proud", which peaked at a modest forty-five on the R&B singles chart. The song was uncharacteristic of the Vandellas and did nothing to promote the group. On some stations, the flip-side "Love, Guess Who" was played instead.  In 1971, the group scored an international hit with "Bless You" (produced by the Jackson 5's producers The Corporation). The song peaked at number fifty-three on the American pop singles chart (the biggest peak of Vandellas' seventies singles), and number twenty-nine on the R&B singles chart. "Bless You" was their first UK Top 40 hit since "Forget Me Not", with the song reaching number thirty-three there. "Bless You" became a top 20 hit in Canada. It was to be the last Billboard Hot 100 hit single for the group. That record also signaled the end of the Motown era. After two successive Top 40 R&B singles, the ballad "In and Out of My Life" (#22 US R&B) and the Marvin Gaye cover, "Tear It On Down" (#37 US R&B), the group disbanded following a farewell concert, held at Detroit's Cobo Hall on December 21, 1972.  The next year, Reeves announced plans of starting a solo career. At the same time, Motown Records moved its operations to Los Angeles. When Reeves did not want to move, she negotiated out of her contract with Motown, signing with MCA in 1974, and releasing the critically acclaimed self-titled debut album, Martha Reeves. Despite rave reviews of her work, neither of Reeves' post-Vandellas/Motown recordings produced the same success as they had the decade before. After living what she called "a rock & roll lifestyle" of prescription pills, cocaine and alcohol, Reeves sobered up in 1977, overcoming her addictions and becoming a born-again Christian.

Why was Ashford replaced?
they could not reignite the fire that had made their records successful in the 1960s. Among their late 1960s hits was "I Can't Dance to That Music You're Playing",