A million youtube videos discuss this topic with tips, tricks, and hacks; Let's keep it simple and go in order of what I see most common in my line of work to simplify.

Are your Body Fat % low enough to see them?

Current rates as of 2022 put the obesity rate at 36% for adults in the United States, or 1 in 3 adults are obese. There's a category before this: "Over-Weight" and even when we get into the healthy range that does not mean that you'll visibly see your abs. So statistically, if you don't have six-pack abs or are trying to achieve six-pack abs more than likely this is the primary reason. Let's also remember to this point that doesn't mean six-pack abs are always a healthy situation and health is not only your body fat %.

Is your core developed enough to see them?

So you've got your body fat % super low and you still can't see six-pack abs. The visual part of your abs specifically when you're looking at a six-pack you're referencing the rectus abdominis, this has to be developed enough to visually show through the skin. Also, lesser-known the transverse abdominis which works in conjunction with the rectus abdominis depending on spinal alignment, training, and breathing techniques could affect the position of individual blocks in your rectus. This type of training is normally more related to PRI-related exercises, scoliosis therapy techniques, spinal alignment therapy, advanced yoga, zone of apposition, etc.

In other words, learn, understand and train your core at the level you need for your goals.

I have a four-pack or five-pack and want a six-pack

This one I can't help you with unfortunately as the individual cross-sections of your rectus are genetic. Occasionally a lower-body fat% can reveal perhaps a missed set of lower abs but in general, that was up to mom and dad. Go back and check Arnold Schwarzenegger; he doesn't have a set of "six-pack abs" and in his time he was considered the perfect human specimen.

For additional info on current obesity rate numbers: here

For additional info on Body fat%, when abs begin to show, and dieting advice: here