Could be a couple of things. With a beginner uke, often the action is set really high so open chords are hard to play without tripping over the strings. Likewise, you've got to squeeze to get the barre chords to sound, and they're often less loud.
The remedy for this is two-fold: lower the action of your strings by sanding the nut or bridge. As you're just starting out, you probably won't want to do this yourself. Have a knowledgeable friend take it on - if its a cheaper uke, having a luthier do it will cost more than the uke itself.
Secondly, practice. It doesn't feel natural, but keep at it. You'll get stronger at strumming and also at fretting the chords, which will correct some of the volume issue.
What uke are you playing? You may find its more satisfying to upgrade to a better instrument.
