This is normal for different players, guitar & the musical material.

What kind of guitar are you using?  What chords are you trying to play, using which strings, when you try to use your thumb?

In general, your thumb is supposed to stay behind the neck & provide opposing pressure.  But in certain situations it can mute or fret notes on the low E strings.

Watch this video when I talk about the axis of power.

https://www.guitartricks.com/lesson/12188

The idea is that your thumb moves position relative to which notes on which strings you are playing. 

It helps to have a guitar with a relatively narrow neck.  I can play the low E with my thumb easily on any Strat.  But it's pretty tricky to do on my classical with the wider neck.  And that's fine, because the vast majority of the classical repertoire requires fingerings & chords that keep my thumb behind the neck.

But there are rock & blues chords & licks that benefit from using the thumb.  Mostly these are situations where the notes you're playing are all on the upper strings (G, B, E strings).  This allows you to move your thumb to the back side of the neck near or on the low E string.

Watch the beginning of this lesson.  I'm shifting thumb positions as I need to in order to go between the rhythm riff & the lead lick.  When I play the lick I move my thumb to mute the low E.  When I go back to the riff I move my thumb back behind the neck to support my fretting fingers.

https://www.guitartricks.com/lesson/18118

So, you might try playing chords that only require you to fret the upper strings.  And even start the process by placing your thumb on the neck near the low E to mute it, then reach for the fretted notes.

Hope that helps!