For the last four months I have been doing the Insanity workout. During this time I have decreased the amount of food I eat to about 1800-2000 calories a day. I drink the recommended amount of water one needs (about 8 cups) daily, and I never eat junk food. I drink a whey isolate protein after my workout (one 29g scoop), I take the ACG^3 pre-workout formula before exercising (one scoop), I take 12 Censor (a CLA supplement) capsules daily, and I get about 8-8 1/2 hours of sleep every night. Over these four months I went from 195lb to 175lb. I'm much stronger and feel really healthy, and I'm fairly tone everywhere except for my stomach. I exercise my abs daily and they've gotten lots stronger, but I'm not seeing definition unless I grab my excess flab and pull it down so I can see where it is covering up my abs. What's going on? I want the definition, and I'm working really hard every day for it. The abs are definitely there, but how do I get it to look like they are there?
I would suggest keep doing what you're doing. You lost 20 pounds in 17 weeks. That's great progress. That progress will necessarily slow down, though.
Ab visibility is largely related to body fat percentage. My guess is that you're somewhere around 20% body fat (http://www.builtlean.com/wp-content/uploads/2012/09/body-fat-percentage-men.jpg). (I'm assuming you are male from your username.)
Since you're still losing weight on your current plan, it's working for you, it will just take more time.
Lasting longer through ab workouts isn't training ab strength, but ab endurance. If you want to focus on ab strength, you need to progressively load them, and that is perhaps best done by working them isometrically as part of a larger full-body exercise, like the squat (http://startingstrength.com/articles/abs_rippetoe.pdf).
I don't know what your body fat percentage is but you need to have around 10% to have visible abs, mine is lower and I still barely see them.
One way to make your abs more visible is to do actual weighted ab exercises. Most fit people can do well over 20 situps which is no longer stimulating hypertrophy, if you try situps with some weight you may see better results.
I have to go back to my original point which is that body fat is mostly what matters, sometimes people who are not even in good shape have six packs solely because they are skinny, you are clearly doing a good job with your workout (lost 20 pounds) I say keep going as you are but increase the intensity of your workout that is the best way to get results, always keep trying harder.
I thought of one thing you can do but note that this is not necessarily healthy. Once you are around the BFP I specified you can start cutting water weight to make your abs show more. I suggest google searching this or ask another question, I'm not a body builder and don't have any reason to cut water weight so I'm not that knowledgable about it.
What abs exercises are you performing? How does your abs feel after every workout? (I know that Insanity videos come with at least one abs video and in many of the other videos, you are advised to always contract your stomach).
If you are seeing the abs under the "flab", that shows that your abs exercises are working. You do indeed have the abs muscles, but they are still covered by the layers of fat.
Do you know how many inches you've lost around your waist? It's important in order to determine how fast you are losing the gut. For example, I do P90X and Insanity (more Insanity than P90X) and over the past 10 weeks, I've lost over 7 inches off my waist. My abs aren't well-defined yet but they are already showing, even without flexing. I can tell that my flabs are almost gone because I cant even pull them anymore. So, measuring yours is equally important too.
In addition to those exercises, do as much calisthenics as possible. They strengthen your core as well as burn calories. Perform pull-ups, chin-ups, legs up, and reverse crunches. Perform exercises that require you to contract your core and those that require you to raise your legs (such as knee jumping, squatting, burpees, and various plank exercises)
While I don't believe in spot-reduction exercises, personal experiences reveal that some exercises target some areas than others. Do more of those exercises that target your core.
You can also lift weights. Deadlifting and weight-squatting target your core too when done properly.
In conclusion, take weekly measurements of your waist. That's the best evidence that something's happening or not.