First we want to find out what the issue is.
Is your forearm circumference small? if so, your forearms just won't look as big as someone who has a bigger forearm bone. Also necessary to take into account is the length of your tendons contrary to the size of your muscle belly. If you have long tendons and a small muscle belly, your forearm muscles will look smaller.
Are your forearm muscles small? in that case you can actually make a difference, though the results of your efforts will depend on your personal physiology. You can do compound exercises that involve the forearms (ie. anything) - this alone will grow them. You can grip handles hard instead of using your arms as "straps" or "hooks". You can add on forearm exercises at the end of your workouts.
I personally don't do forearm-specific exercises, I do compounds with a solid grip. The reason why is that the forearms already get worked with everything you do that involves holding a weight in your hands which most exercises for the upper body involve doing. That's why when you get tendonitis or something it will most likely be at the elbow, forearm or wrist, so not only is working them directly probably unnecessary (unless you just want big forearms), it may negatively influence other movements because of fatigue and increase the risk of something bad happening which stops you from working out safely.