With no disrespect intended, if your coach is telling you to throw combinations as you described he is misleading you. If you are familiar with Juan Diaz you are aware that he throws a ton of punches he can't break an egg. This is because he punches from a set position- in his last fight he'd be leaning unto his left leg, start a combination and throw left/right/left/right etc...5,6,7 blows. All of the time on the front foot which is why he lacks any pop in his punch. When you throw a hook a right hand or an uppercut the force comes from the twist at the hips, which is only possible if the wt is shifted from leg to leg. That is also how balance is maintained.
It is common to vary the force of punches in a combination- if you've hurt him to the body for example the hook down stairs is the one you may choose to set down on. The others you just snap out with brisk wt shifts from foot to foot.