Finishing power

I have a problem with reaching back and getting a little bit extra to finish a combo, like it isn't so much I can't punch hard, I just can't punch a tad harder when I need it. I was watching a tape of myself fighting, and I think this cost me two knockdowns. I staggered my opponent once, but I couldn't put him down. Another time I hit him with a good combo, five solid blows right in a row, again he was stunned but I didn't have anything extra to finish him with. Is this technique, pacing, or just a problem I will eventually overcome as I get stronger, faster, and better overall? Thanks.

Thanks, after I thought about it I was afraid someone was going to reply "Sounds like you're just a wimp."

Yeah, I have terrible footwork. I can't get past my "I've got long arms, why get any closer?" mentality.