I recommend shaving more often. Shave it bare, let it grow back just past 5-o-clock-shadow stage and then shave again.

When you shave, you splice the hairs in awkward ways, making them thicker and filling in the patches.

I have had these issues myself and ever since I made an effort to shave more often like this, I have noticed major improvement.

The only downside is, I don’t like having NO beard, so I had to get used to being shaved for a while before I was able to grow in a fuller beard.

Hope this helps.