Guna milan is not complete horoscope matching , you should do a complete Horoscope matching which means Guna and Grah milan analysis. Also the strength of the 7th house of BOTH horoscopes is very important. If one horoscope is weak and the other horoscope is strong, the strong horoscope will provide support to the weak horoscope. If both horoscopes are weak then surely the relationship will not work.
Blessings,
Navneet Khanna
Astrologer