Yes, that’s a common issue. You’ll likely find it even more of an issue when you get to the minor 7th chord.
It is partly strength, but also positioning the hand properly.
First I’ll recommend the you go back and work on the bar chord exercise that I teach in the course. Do it twice a day for a week… just 5 minutes or so each session. Listen to each of the notes as you do the exercise and work to get them to sound clear.
Here’s another version of it: http://www.tomasmichaud.com/heres-secret-learning-bar-chords-sunday-update-30/
Second, after you’ve done that for a week… do the same exercise everyday for another week with one change. Use the 7th form of the chord instead of just the bar.
Let me know how it goes.