How to strum hey soul sister on acoustic guitar?

I know how to play all the chords on the guitar for Hey Soul Sister by train and everything but the strumming is the problem.....im sortof a begginer.Everytime I strum a not like for example,If I play the Gmajor chord and strum it will come out terrible and you cant even notice that the chord is beign played.I've even tried pressing down on the string harded but the same thing happened,and I also made sure I tuned the guitar if that helps....