your jab is probably harder than you think... most people don't get snap on the bag with their jab. it has to hit either very hard or so perfectly that optimum surface area comes into contact. mine doesn't snap the bag either yet people tell me my jab is hard... but I think it sucks for reasons similar to you. it just doesn't "sound" hard. But in reality it's fine. Not my best punch, but it gets me by.

just keep working on it... strength training, in the mirror, etc.

not to call attention to myself, but did you see my video. I had the very same question as you when I started.. how to strengthen my jab. so far my pulley system has made a lot of difference in all my punches.