Mars being small means its weak gravity isn't able to retain a thick enough atmosphere for life, the solar wind keeps blowing it away. There is a thin atmosphere though, note that you can't see the stars when watching video footage from the rovers there. It's caused by frozen water deposits at the poles leaking into the air, and by the dust being blown around by the terrific winds.
I read about this a while ago, so may well have forgotten or misremembered bits.
So, two problems: Mars isn't geologically active, and it's too small. Solution? Crash an asteroid into it, preferably one which has frozen water deposits. Do so at such an angle that Mars' orbit will be shifted to be closer to the sun when it stabilizes. Then, possibly, the added mass, added water, closer orbit, and volcanic activity might make it possible to eventually terraform Mars to be a sister to Earth.