Psi attack/defense is orthogonal to conventional weapons. Its result depends on Morale levels of attacking/defending units.
If an attacker/defender is a Mind Worm, they have their own class, plus both faction's Planet (Green) scores/attitudes largely affect the outcome of the fight.
Answer: see what's your faction's Morale and/or Green score.
You may also trick the system by changing your Government/Economy type before the attack. Say, if you possess some Mind Worms and you are planning to give them a victorious ride, switch to Green several turns before the planned invasion. Or to Fundamentalist + Power if you are planning Psi attacks with conventional units.
Personally, I love Green because if you are lucky enough, you can capture native life forms, making a considerable amount of your units Mind Worms (Independent) which means it requires no support from a home base, still performing as police in "at least one unit defending each Base" paradigm.