The most common point where rhythm breaks down for guitarists is during a chord change. While your fretting hand is looking for the correct shape and the eye is focused on the fretboard, your strumming hand sits in the air until things feel “settled”. This can become a habit, even if the chord change was correct. When this happens, the music does not sound as steady as the fingers suggest, because there is no movement on the beat when the chord has been changed.
Imagine that the strumming hand is the instrument that is responsible for playing the pulse of the song. It does not need to strike the strings with every motion, but it should always be aware of where the beat is. If the strumming hand only moves when the chord has been changed, rhythm is completely dependant upon how quickly the chord change occurs. That creates an emergency for every single chord change.
To start learning this skill, take a chord that you know, lightly mute the strings with the fretting hand and use a single downstroke as the strumming hand moves in a slow, continuous motion. Do not worry about making any sound while counting one-two-three-four out loud. The goal is to create a relaxed motion coming from the wrist/forearm that can be sustained without the chord change prompting that motion.
Next, take an easy open chord and strum on count one, keeping your strumming hand moving continuously on the other counts, even if you do not make a sound on those beats. On the next measure, try a different chord. You will not get the change in time at first, even at a slow tempo. But it is always better to make the mistake of being late when you know the timing is off, rather than hiding your timing by stopping the strumming hand.
Some beginners will attempt to fix this by trying to increase the speed of the fretting hand. This can lead to the fingers lifting high, striking too hard, or pressing down too far from the fret. A more relaxed approach is simply to play the changes slower and keep the strumming hand moving continuously, regardless if the chord was changed in time or not. The strumming hand can gently touch the string or miss the strings entirely, the only objective is for the rhythm to continue.
You can also use a metronome as a useful reference for this practice. But be careful to only increase the tempo when it feels comfortable. Using the metronome should not add pressure to your playing. If it does, you need to slow the tempo to feel more comfortable counting with the metronome and moving the strumming hand in a steady motion. Once you have the chord changed, the rhythm should continue without interruption or stopping until the next change.
Try to listen to your guitar right after a chord change has been made. Does the strum sound too loud and fast? Or, does the chord sound too muffled? Perhaps the fingers landed too flat or too far back from the fret. When these issues happen, try going back to the muted string exercise, and simply focus on creating continuous motion in the strumming hand. This will help to create a relaxed motion that is driven by the tempo of the metronome and not by a chord change. A good indication of this progress is that your strumming hand is no longer tied to panic. It is constantly moving because the tempo has changed the beat, not the chord. There will still be a lot of buzzing as the fingers land correctly in time and place. Keep this practice slow, steady and continuous, and you will start to feel the chord change fitting into your rhythm rather than interrupting your rhythm.