After getting a wall stun from a grounded position, in order to get a re-stun, you need to be able to hit with an attack within 20 frames of the initial attack hitting. Other than moves where you can cancel them immediately after the hit connects, like Bryan and Xiaoyu's taunt, it's very difficult to achieve this, except in strings.
Also, even if you satisfy the numerical constraints, if the move doesn't connect with the opponent of course it won't work.
Yoshi's d/b+2 and Bryan's b+2,1,4 etc. because of their high hitboxes are difficult to combo.
After the wall stun occurs, between frames 21-30 the opponent is invincible, and between frames 31-33 they are considered grounded and can eat some attacks. This down state is larger than the normal down state, and some moves that have relatively low hit boxes will unexpectedly connect. (Bryan's ws+3, Feng's b+1+2). For the already large Marduk, Jack5 and Kuma this is quite dangerous. This condition is coming to be known as the "non-ratio'ed" condition.
During the "non-ratio'ed" condition, as the name would suggest, there is no reduction in damage for the hits, so all you have to do is choose moves that wil hit within that time frame. The way to choose a good move for this is by looking at the frame tables. Generally, after a move that leaves you neutral on guard, a 12-14 frame move will connect within this "non-ratio'ed" time. (Moves that have a unique guard animation are exceptions.) Of course, there are strings that will already satisfy these conditions, and these are easy to apply (Lee's 4,3,4 etc.)
Something to pay attention to is the execution frames but also the number of active hit frames. For example, Yoshi's f+1 is +3 on guard, so you would think that f+1+2 would connect, being i17. But, the time within the 31st-33rd frame is not all the same. With each frame that passes, the hit box becomes slightly narrower. This move will hit on Marduk, Jack5 and Kuma on the 3rd frame, but will only hit on normal sized characters on the first frame. So, if f+1 (+3) is impossible go for it after d/b+2,2 (+6) instead. The reason that f+1+2 will hit after d/b+2,2 even though frame wise it isn't coninciding exactly with the 1st frame is because its hit frames overlap that point.
However, if your opponent inputs the command for a techroll, it would seem that the hitbox across the 3 frames is the same as that for the 1st frame, so in the above case if you connect a f+1 after a wall stun, you could hit a f+1+2 if they choose to tech.
This contains a lot of theory, but it seems to hold in real situations.
It was said at the start that to restun after a wall hit you need to connect within the first 20 frames. But, in actuality after the first 4 hits the time in which the opponent falls is shortened by 5 frames. After this, you need to connect an attack within the first 15 frames, or the restun will not occur. This is the reason that Law's, Ganryu's and Roger's repeated attacks only continue to the fourth hit.
Of course, this shortening of 5 frames also has an effect on the "non-ratio'ed" time. On the ground, d/b+2,2,2 1+2 will connect, but d/b+2,2,2,2 1+2 will be impossible, becase the hit window is 5 frames earlier. Devil Jin's 1+2,2 (+0) d+3+4 (i14) will connect, but 1,2,1+4,2 d+3+4 will not.
