The reason some people need to take more medication for the same effect is because you can become tolerant to the medications. If your body consistently sees the medication on a daily basis what it does is it stops reacting as it did before so for the same amount of medication you may have less effect that you did maybe a year ago.
In order to avoid that sometimes you can end up switching the type of medication that you provided someone so instead of giving them compound A you may give them compound B which throws the body for a loop and you have the same effect as it did before or you may have to sometimes increase the frequency or increase the strength of the medication to have the same previous effect.