If you're feeling a hard end of range of motion, rather than a soft one, there may be a problem that should be checked out by a doctor. That said, to increase ROM you normally need to stretch the five adductor muscles of the femur: the pectineus, adductor brevis, adductor longus, gracilis, and adductor magnus. Stretching should be slow and steady, and avoid "bouncing" at the end, which can tear muscle fibers and lead to the formation of scar tissue. (How the muscles feel during the stretch is what's important, not any particular distance or angle of stretch.) For specific guidance, I'd suggest looking on Amazon for an old book by Bill Wallace called 'Dynamic Stretching and Kicking.'
Yours in Budo,
---Brian---