Weak strikes.

[SaiFightsMS]

I think it is a rare for a beginning student to not have trouble with power.

[Kirves]

Make sure your form is good (use hips, legs and back to generate power). Hit the bag, hit mits, do some speed push-ups (like clapping hands together during pushups) and so on. Build stronger muscles via weight training.

[granmasterchen]

number 1 thing is to get your skill level at the right area, make sure that you are punching correctly, if you like try knuckle training exercises, hitting heavy bags are good, also do griop training, finger tip puchups and knuckle puchups, practice hitting hard objects to build up the level of resistance this will increase muscle mass,also try to hit 6 inches beyond your target(hit through your target) this will help also,make sure that you pivot your feet whit your punch, get the skill down.