It's an interesting shame that singer-songwriters those wandering minstrels who work so hard to make sweet-voiced, fancy-footed, high-polished superstars sound like they have more dimension than their cup size must work so hard to get recognition. John Prine's folk beginnings in the 1970s were that particular mixed blessing of being a darling of fellow musicians but missing the public mark entirely. Kris Kristofferson pushed Prine out into the world, and his songs were soon covered by the likes of the Everly Brothers and Bette Midler. It wasn't until Prine got into rockabilly and in the '80s, started his own label, Oh Boy Records, that he took home his first Grammy for "The Missing Years." He won another Grammy this year for "Fair & Square." After recovering from throat cancer, he's not sweet-voiced and he's not high-polished, but now everybody hears him. He plays the Landmark Theater April 21 at 8 p.m. Tickets are $29.75-$49.75. Call 262-8100 or visit www.ticketmaster.com.