The Incredible String Band (sometimes abbreviated as ISB) were a psychedelic folk band formed by Clive Palmer, Robin Williamson and Mike Heron in Scotland in 1966. The band built a considerable following, especially in the British counterculture, notably with their albums The 5000 Spirits or the Layers of the Onion, The Hangman's Beautiful Daughter, and Wee Tam and the Big Huge. They became pioneers in psychedelic folk and, through integrating a wide variety of traditional music forms and instruments, in the development of world music. Following Palmer's early departure, Williamson and Heron performed as a duo, later augmented by other musicians.
Diminishing returns: 1971-74
After that the group lasted another four years, although there was a gradual decline in their status and commercial success after 1970. Joe Boyd, whose skillful handling of the band had contributed much to their international success, stopped managing them and returned to the US. The group left Elektra Records and signed with Island, for whom they recorded five albums. The first was a soundtrack to the "Be Glad..." film, and this was followed by the eclectic Liquid Acrobat as Regards the Air, regarded as their best album for some time.  The band continued to tour and record. Rose Simpson left in 1971 and was replaced by Malcolm Le Maistre, formerly of the Stone Monkey troupe. Mike Heron took time out to record a well-received solo album, Smiling Men with Bad Reputations, which, in contrast to the ISB's self-contained productions, featured a host of session guests, among them Pete Townshend, Ronnie Lane, Keith Moon, John Cale and Richard Thompson. The following year, Licorice left, and was replaced by Gerard Dott, an Edinburgh jazz musician and friend of both Heron and Williamson who had contributed to Smiling Men. Williamson also recorded a solo album, Myrrh, which featured some of his most extraordinary vocal performances.  The group's changing lineup, adding Stan Schnier (aka "Stan Lee") on bass, Jack Ingram on drums, and Graham Forbes on electric guitar reflected moves toward a more conventional amplified rock group. Their final albums for Island were received disappointingly, and the label dropped them in 1974. By then, disagreements between Williamson and Heron about musical policy had become irreconcilable, and they split up in October 1974.

Q: what happene din 1971?
A: The band continued to tour and record. Rose Simpson left in 1971 and was replaced by Malcolm Le Maistre,
why did he leave?
Mike Heron took time out to record a well-received solo album,
Mike Heron took time out to record a well-received solo album,