Abdominals are located between pelvis, and rib bones. So if you are flexing hips - that is not perfect move. By that I mean you are not isolating abs, and training some other muscles - like long leg of quadriceps - as well.
Or maybe I'll start from other side - abs are responsible for stabilization, and something called abdominal compressor - here is sample article, with other references. So they are used, and trained in different situations, however it is good to have isolated exercises a specially if you are targeting that particular group.
There are plenty of exercises you can do, perhaps you already know them, but I'll put them here to have consistent answer. There are - twists, with plate, plank, cable crunch. I mentioned isolated, but there are others as well. If you are training so versatile muscle group be sure to use different exercises. Mainly that is why I've putted that references.
About plan - there is no plan that would work for you forever. Key is to change it, but also not too often - block should take 2 weeks and at least 2-3 that blocks makes a phase. So let's say you are working on definition for month or two. With smaller blocks, which allows minor adjustments. If writing about plan... I wonder how to track progress. Number of reps, or time in plank? But that is not your goal - you want it big.
For posture muscles better effects can be achieved by bigger number of reps, with quite low weight which goes quite well with training at home. Also isometric exercises like plank are good.
Other typical advice is to have one or two sets for targeted group at start of every training. That would increase load, so probably you can gain from that. Why probably? Because genetics can go against you. If you are doing a lot, and you can't get more - maybe that is the problem. Please find trainer, or physical therapist, someone skilled - to talk face to face. At least that way you can know if that is the problem.