Short answer: Increase volume of lagging muscle groups.
More nuanced: You can do this in a variety of ways, but it must also be quality of volume. You have to determine why the muscle is lagging. Simplest approach is to increase the volume on the lagging group while decreasing the volume (more to a maintenance level) on other muscle groups. This could mean a variety of things depending on your split, but I usually up the frequency of the target group (so say you train 5 days per week you could do M-Upper, T-Lower, W-Arms, Th-Lower, F-Upper, if say Arms were lagging).
Now again you must keep in mind why they are lagging. If it is due to poor activation of the target muscles (say you can't get a pump, you never feel that muscle "activating", you just can't seem to progress on the weights with them) then just piling on volume may not help, but a more targeted approach to improving quality would be in store.
You could also prioritize the muscle in a session. Train it first if it is normally later, etc.
That is far from comprehensive, but that is my usual "approach" when in doubt.