Make sure your holding you guitar plectrum or pick correctly.
Your wrist should be relaxed and free to move. A common mistake is to hold the wrist very ridgid and then move the pick or plectrum by moving the whole forearm.
To get a fluid and pleasing strumming action the majority of the motion will come from the wrist.
Remember remain relaxed and don’t tense the wrist otherwise you will end up strumming like a 1980’s robot style dancer.