This can be a number of issues but the root cause usually comes down to discipline.

You MUST keep your hand moving!  Pauses at the top or bottom of your strum are a common problem.  If your hand stops moving, you’re doing it wrong.

A lot of times students have an easier time strumming down than up.  Other times you might strum faster when you are actually strumming the strings than when you are skipping them.  Either way, they need to be even.  Keep your hand moving at the same pace throughout your entire pattern!

Make sure you are strumming in a straight vertical line and hitting all the strings.  Strums sound inconsistent when your strum path looks like a crescent moon.  The pick gets caught up on the middle strings.  You miss your bass notes.  These are all bad things.  Strum straight through the strings every time.

Tips:  Hook yourself up to electrodes that shock you every time your strum hand stops moving.  Even better, just take it slow and speed up in increments.  If you start falling out of time, slow it back down.  If you don’t get strum technique down in the beginning, you will be fighting it for years.  You don’t want to be this guy.